Tube Amplifier Sound Characteristics

I wish there were a scatter diagram one could put together of tube amp sound qualities. The reason I say this is that today's tube amplification can range from sounding warm and romantic to cool and solid state like.
I like the sound of VTL, Quicksilver and Antique Sound Labs. I am in the market for a tube power amp and as I shop I see where more and more amps do NOT sound like tubes at all. So where would the sound characteristics of the 3 amps I mentioned be? For example I was considering the ARC Ref75SE but owners tell me it is SS sounding. Your thoughts are appreciated.
 and more amps do NOT sound like

All tube amps sound the same to my ears, 34,88,120,250 all sound pretty close.
Tube = tube
Minus the 300b which was made for telephone usage = dull
Heard several tube amps, there are/subtle differences, while others sound completely different. 
@miillercarbon maybe a liner scale lol. On the left is warm romantic, on the right cool neutral?
@russ69 My speakers are DIY Satori MW16P (x2) and TW29R tweeters. 2 way design. 90/91 db efficient, -3db @48hz.
BTW since I have heard the VTL and like their sound I would buy one on the used market except that only the really old ones are available and I am willing to spend more to get something not too old.The ASL are nice sounding but poor reliability from what I have seen. My tech advises against it.
I like the ARC but one owner tells me it is on the cool side, a bit bright and edgy.
Doesn't the "sound" of a tube amp depend on the various tubes involved?

If there was going to be a Venn Diagram, it would have to be at least two Venn diagrams -- one comparing different amps with the same tubes, and one comparing different tubes with the same amp.

How much feedback is being used. What do you know about the transformer windings. I know some from a certain country do about 6 windings. With a machine. Terrible frequency characteristics, but hey, it looks pretty.

Once you are satisfied with "basic" build features such as these and perhaps others will become relevant - tube rectified? choke, any ICs, SET or push/pull, cathode/fixed bias? etc. (SE *may* mean its a SET).

Then what tubes? 300B are a very different beast than EL34s, for example (try to stick with popular tube types such as these).

Other things you might ask about, to get an idea if the vendor has more than 3 neurons - is it a triode, ultralinear, pentode or other type of amp?

I know nothing about brands that come out of a factory.  Mine was handmade by an artisan.

Be informed.
There are a lot of variables in play....the circuit, the caps, the tubes, the rest of the system...all contribute, so even an amp of the same brand and model can sound different with different tubes.  One of the things I like about tube amps is that you can tailor the sound to your liking by carefully choosing which tubes you use.  
+1 @knotscott  Spot on. Sort of makes most of the threads debating about amp brands look suspiciously like a waste of time.
@Hilde45 I was just thinking of tubes as supplied from factory or stock units. For example on the extreme left (warm/tubey) I find Conrad Johnson. I have a Rogue preamp that is more in the middle. I have heard from people that in general that is their house sound.
I would caution anyone thinking about older VTl....yes they sound good, but if the day ever comes that you need service you will experience the worst customer service on the planet.   
@oddiofyl I have heard, that is why I am reluctant to buy any older VTL. I do have a good tech to fix amps but I dont want anything too old.
Like any topic, it all depends on how much you want to dumb it down on the spectrum from peer-reviewed white papers at one end and Twitter posts on the other.
With so many variables at play I don't think a simple warm-cool linear scale is enough to make an informed decision. It might let you cross a few brands off a big starting list. 
A more useful but still digestible format might be similar to how some reviewers rate ~10 attributes and score each on a 1-10 scale. Even rating a few amps you know would take some work.

Honestly, I think familiarity with the nature of various tube types is one of the most critical aspects of what OP is trying to solve for, maybe more than the brand name. Joe's tube lore is a good source for small signal tubes, but I don't of anything similar for common amp tubes. 6550s, EL34s, OTLs w/6AS7s, KT88, KT120, KT150 and little EL84s all have their own characteristics that are a big piece of the puzzle. Sure, circuit design, other parts quality, etc. also contribute. 

Choosing tube amps without knowing tube types is like reading the Torah(they don't print the vowels!). It can be done, but is a lot easier if you understand. Cheers,

On the left is warm romantic, on the right cool neutral?

Okay having studied your requirements the solution is obvious. Take a clean piece of paper. Draw a rectangle about 5" wide by 2" high. Label the left, "warm romantic". Label the right, "cool neutral". Take a crayon, color this blob yellow. This is your tube amp diagram.  

Now we do the same for solid state amps. Let's use blue for solid state. 

Now if you followed the instructions well you should be looking at a green rectangle. Smack in the middle draw a great big letter "S". Now divide the "S" by drawing a big ol' vertical line right through the middle.  

This is where you will find your amp. The more of these $ you make the better the amp you will find. 
OK I should have stated that I am new to tube power amps. That explains a lot of my ignorance on the subject and I apologize. I know what I hear people saying and about the sonic characteristics of different tube amps and was trying to identify those on the warmer side. So if it has to do with the tubes I know EL34 in triode is warm/romantic. I will read up on KT120, KT88 and 6550
I love the idea of a chart locating the sound characteristics of different equipment! One could start by locating house sound on different dimensions. Tonal balance, detail, warmth… great idea.

The ARC Ref75SE is a very high end and much loved amplifier. It does not sound like solid state… on the other hand, the moment you hear it it does not broadcast “I am a tube amp”. It has the house sound of Audio Research. It is a high end audiophile amp that is musical and accurate. It is highly detailed with great and strong mid-range bloom and articulate bass. I know of a number of people upgrading from that amp, but can’t bring themselves to let it go. Unless you are looking for syrupy bass and rolled off top end then this is a fantastic amp which you could easily keep for a lifetime.
Click on my user ID to see my systems… note there is some Audio Research gear in it.

Also remember the sound you get out of your system is the sum of all components.
Everyone focuses on the tubes, because they are so obvious and easy to change- and there are indeed differences between tubes. But tube amps really are no different than anything else in that everything matters. Equally important as tubes are transformers. Equally important as transformers are diodes, caps, circuit design, etc.  

If tube amps were made with diodes that plugged into receptacles on the top that could be changed by hand as easily as tubes then mark my words everyone would be talking about diode rolling and the virtues of amps build around their favorite diodes. Because I have swapped and upgraded diodes and believe me the difference is huge and obvious and can take a flat grainy edgy amp and make it liquid warm and deep. 

In other words it really is just like ghdprentice says above, a system is the sum of all components. Only thing to add, this carries straight on through to the components used within the components.
OK I should have stated that I am new to tube power amps. That explains a lot of my ignorance on the subject and I apologize.

Don’t apologize for what you don’t know....especially when you’re posting questions to learn.

One of the common themes I find with decent tube amps when utilized with a revealing system, regardless of the tonal characteristics of a particular tube, is the resolution and clarity through the mids and treble that creates a more palpable convincing soundstage that transforms me from the couch to the recording session....obviously it does vary from amp to amp, and tube to tube, but that's one of the stronger general traits that draw me to tube gear.  Many tube amps don’t have overpowering authoritative deep bass compared to a high watt SS amp (some do), but many have outstanding micro dynamics that create more concussive transient attacks from instruments like strings, piano, percussion and drums, guitar plucks, etc., that are part of that convincing transformation. Even my $250 Nobsound amp had some of those characterics....beefier, more prestigious amp circuits do also, but even more so. At least that’s part of what I hear from my tube amps that keeps me craving the sound of tube gear.
All amps sound different to varying degrees.
There is that.
Then you express a desire to find the sound you like. Only you know.
There is that.
A chart might be a good idea depending on the sonic integrity of whoever makes it.
Thank you @knotscott. I demoed 3 amps in my system in order of preference VTL, Quicksilver, Carver. I dont need thunderous bass. I want naturalness of strings and woodwinds/brass. I actually like bass that is a little bit round. I run GR Research servo subs so they are tight tight tight lol. It's funny when I got my RP7 out of the box it was very cool sounding and I remember myself saying "If I wanted SS I would have bought SS!" haha. I retubed with NOS Baldwin black plate to tone them down a bit. I had someone who owns a Ref75SE say it sounds like a SS amp.
A well-designed amp, tube, SS, whatever, should not have a particular characteristic related to it's underlying circuity.  They should be honest to the music and reproduce sound without coloration or other distortion.  SS sound or tube sound just describes distortion.

Unfortunately, some people enjoy distortion thinking it sounds "musical" or some other bs term.  Whereas true musical reproduction means reproducing the signal AS RECORDED without distortion.  
"Unfortunately, some people enjoy distortion thinking it sounds "musical" or some other bs term. Whereas true musical reproduction means reproducing the signal AS RECORDED without distortion."

I have seen this position stated before. As far as I'm aware every amplifying device has a signature or unique sonic fingerprint/character. This includes all tubes and transistors.

 Hi @bpoletti what specific amplifier or audio component are you referencing that is capable of reproducing "signal as recorded without distortion" ? I'd love the opportunity to hear them.
@ bpoletti

A well-designed amp, tube, SS, whatever, should not have a particular characteristic related to it’s underlying circuity. They should be honest to the music and reproduce sound without coloration or other distortion.

And yet none (or almost none) actually achieve this. They all fall short of the theoretical ideal, regardless of how well they measure.  I think the key more accurately lies in where and how the distortions occur, because some amps are simply more convincing at portraying a music signal while driving a speaker.
Listeners at some point in the decision making process choose the type of distortion characteristic they find the least objectionable. There’s no escaping some degree of distortion from electronic audio devices. High odd order harmonic coloration or low even order harmonic coloration. I’m interested to read what amplifiers that @bpoletti can cite as examples. A thoughtful topic.
I wish there were a scatter diagram one could put together of tube amp sound qualities

In a  blind test, you might not determine which is tube which SS, but trust me, tube is the only way to hear music.. Nuances nunaces nunaces
make sure front end has 12AU7 section,, when I swap brand X for a  Telefunken 12QAU7,, I note a  drop in sonics,, So with tubes you can have fun rolling, + rolling different power tubes,,make sure the tube amp takes all varities of power tubes.
EL34 = KT120 bias..
Basically all power tubes sound same, but its fun rolling anyway.
@mozartfan my pre uses 12au7. Ive never used telefunkens in them. I used Baldwin black plate now.
I wish there were a scatter diagram one could put together of tube amp sound qualities.
@jimbones The last thing the industry wants is for the buyer to know what the amps sound like :)    They spend a lot of time trying to tell you that they don't sound like anything at all.

But there are differences and if you have enough information you can figure out what the sound will be to a certain degree.
So here we go:
In all cases keep in mind that this all about linearity.

There are differences in power tube types roughly split between triodes and everything else. Triodes are more linear and so will sound smoother and more detailed. But there is Ultra-linear operation (an output transformer thing), which if properly executed, results in a pentode or tetrode having triode linearity as well. The key in this latter bit is 'executed properly'. Many OPTs (OutPut Transformers) have the screen taps improperly set to avoid a patent infringement of long ago, and that's become a bit of a tradition- often designers don't question the taps.

You have single ended VS balanced and a combination of the two; the latter of which is most common. Single ended circuits have a quadratic non-linearity which causes them to make a fair amount of 2nd harmonic distortion. Balanced (often differential) circuits have a cubic non-linearity, which causes them to make a 3rd harmonic (in theory the even orders are cancelled throughout the circuit, resulting in dramatically less distortion as distortion is compounded less from stage to stage), with succeeding harmonics falling off at a faster rate as the order of the harmonic is increased. The ear treats the 2nd and 3rd the same- they contribute to the 'warmth' of tubes.

So an SET can sound quite warm and vivid because of its distortion. It can also sound dynamic on the same account if you are not careful about the speaker selection. A fully balanced amp will also have some warmth but will sound more transparent owing to reduced distortion. Because of the reduced distortion, it will also sound less 'loud' at higher volumes.

To give you an idea of how profound the difference is in distortion, almost any SET will produce 10% THD at full power which might be only 7 or 8 watts. A fully balanced amp using the same tube types might produce 1% THD at full power, and full power will be about 4x greater if the same power tubes are employed. So at 1 watt (keeping in mind the first watt is the most important) a balanced amp can have well over an order of magnitude less distortion. This is heard as clarity.

If the amp has single-ended input circuitry combined with a push-pull output, the two non-linearities are present and due to algebraic summing, the 5th harmonic becomes more prominent. Most tube amps fall into this category. The 5th isn't a good thing- this makes the amp a little less musical.  

The class of operation can have an effect. Class A amps generally are more linear and so make less distortion. IOW they will be smoother and more detailed. This is not a hard and fast rule; if the designer is careful a class AB1 amp can have low distortion as well; the designer has to be more careful about preventing distortion from cropping up when the signal is crossing the 0 point between positive and negative.

Generally speaking, the more complex the circuit the greater the places things can go wrong such as the effect of additional coupling caps, loss of bandwidth, increased distortion and noise.

Finally feedback plays a role too. In tube amps feedback is useful in getting the amp to behave as a voltage source; in most amps this would be about 15 dB of feedback (feedback is the practice of taking some output signal and applying it to the input of the amp, out of phase with the input signal so it can act as a correction voltage). The problem here is that feedback in such an amount adds higher ordered harmonics of its own; any amp with that or a similar amount will be harsher and brighter (more 'solid state' sounding) than the input signal. This is because the ear interprets the higher ordered harmonics (5th and above) as harshness and brightness. Such amps may sound 'loud' as well, since the ear uses those harmonics to sense sound pressure, so they will impart a sense of loudness. A sound pressure level meter will show the truth of the matter.

So now you have a rough gauge of how amps will sound if you can get enough information about them; triode amps operating class A will be at one end of the 'smoothness' spectrum and class AB tetrode or pentode amps with push pull operation and a single-ended input, with feedback will be at the other end. However in some cases the latter will also be more detailed as they will have lower overall distortion. The one exception is that if you can find a balanced triode tube amp that lacks feedback you'll get both smoothness and detail, since this type of amp tends to have the greatest linearity overall (whether feedback is used or not- again, feedback will increase brightness and harshness).

One variable of all this is output impedance. Most speakers are designed to expect that the amp will make constant voltage regardless of the speaker load. For that you need a low output impedance and the only way you can get that is by adding feedback in a tube amp. So the speaker choice can have a dramatic effect on the results you get. A simple tip, regardless of the amp: Avoid Four Ohm Speakers. They will cause any  amp to make more distortion and with tube amps also less power and if the amp employs an output transformer (most do but there are exceptions known as OTLs), a loss of low frequency bandwidth by as much as an octave. In a nutshell the wrong speaker can shoot down the good qualities of **any** tube amp! So you have to be careful about that if you want to get the most out of your amplifier dollar investment.
Teaser! I don't want to brag but just got new integrated amp that can run circles around the usual suspects! Last time I alerted giant killer was Stenheim Alumine three!

Enjoy the music!😆
@atmasphere Thank you for the thoughtful reply. I love your statement that The last thing the industry wants is for the buyer to know what the amps sound like. They spend a lot of time trying to tell you that they don't sound like anything at all. So True!!

But as we all know sometimes when we insert different equipment into our systems, the sound changes. I have to admit I don't know what "true neutral" is. How would I know? I admit I love distortion, not sure I can tell you exactly but when I hear it I melt. Me personally find that the tube power amps sound better TO ME. Thats all that matters if if I like what I hear whether it is even or odd order harmonics.

Too bad my speakers ( My DIY design) are 4 ohm. I guess I can build an 8 ohm version.

@kw6 I agree the integrated are really good. I remember last year I was at a audio dealer showroom and he said he was putting on an integrated and I snickered and then heard it and was amazed how far they have come. I personally think it is the wave of the future as the demographics and industry have changed.
But as we all know sometimes when we insert different equipment into our systems, the sound changes. I have to admit I don't know what "true neutral" is. How would I know?
@jimbones The only way I know isn't easy. Get some great microphones and record an ensemble of some kind- and then have it on whatever media you prefer. That way you know how it should sound because you were there when the recording was made.

Otherwise you play natural recordings that have good merit and listen for the things that they are known for. As those merits improve (and lessor recordings don't get worse) when you change a component, you'll know you are on the right path or pretty close.
Man, I love when @atmasphere joins a discussion! I really appreciate learning the “behind the scenes” details from someone who truly KNOWS! I don’t have much to contribute in terms of what one hears as a result of electronics design, but I can say this: the only way to know what a component sounds like, is to insert it into one’s own system and listen. That seems obvious, and it is stated here often, but its importance is still undervalued. “You can’t judge a book by it’s cover” (or a component by it’s circuit), is very appropriate in audio. I’ve been surprised many times by how something sounded, as a result of my expectations suggesting one thing, but then reality showing me those expectations were wrong. And that goes both ways: expecting great, but hearing mediocre.. and expecting mediocre, but hearing great

as was said, if someone made a graph of sonic attributes for any amp, SS or glass, the author would regularly be challenged.

at least to my ears, lately it seems more and more tube power amps tend to sound similar to Ss amps.

When picking any amp it seems to me one should enjoy what it provides in its unadulterated form, first.

I want a tube power train usually as most often the outcome is more pleasing than what one routinely gets from pure SS amps, AND with Tube power one can address their audionervosa from time to time by rolling the dice and spending far more money and replacing what the designer felt were optimun tube choices at the onset of their construct.

always I want a tube amp to sound unlike an SS amp… mostly. as the result of distortion? of course not. ultrra euphonic? nope. not that either.

what I find preferable with tube power done right (IMHO) is a fuller, more dimensional more organic presentation which enlists leading edge definition and does not ignore the natural decay of tones. one where imaging is as prominent though no more so than the balance of its delivery of the entire bandwidth.

… and I like it to be a little bit wet sounding.

regardless one’s preffs this past time is about personal involvement and enjoying the music being rendered.
if the end result floats your boat, butters your bvread then all other reputed soothsayers be damned. you are the one who has to live with your rig… not them.

as for tube poewr amps that really grabbed me, they were not power houses per se. often rated at less than 60 watts per ch max, and usually less.

as for a subgjectivve account of the OP’s short list, I’fve only heard the VTL and ASL. both were in completely different set ups and different rooms. my only take away was the ASL amp array yielded a more prominent mid range whose protrayal came across as more solidly done.

there are far too many variables in compariing amp attributes let alone the subjective assignements each poster will attach to their comments to develop a valid consensus.


Absolutely tube amps and solid state amps have been converging… particularly in the late 90’s and early 2000’s. You used to be able to tell what it was instantly. Solid State has gotten less brittle and harsh and tube equipment has gotten more detailed and neutral. The difference now can be subtle… typically still there. At high levels of performance it can still be a very important difference.
Amplifier sound boils down to topology and parts selection. The two extremes of topology will give either the most cold or most warm sound:
A. Push-pull design, class B, high feedback = "solid state sound", even when it is a tube amplifier.B. Single ended class A low or no feedback= "warm tube sound" (=live, as opposed to dead) with either tube or solid state.
Parts selection - a very long story, will not attempt it in a few lines.
However, it's the parts that will define how your chosen topology will sound like:So, the shapes that take form can be (from worst to best):A. the cold sound into any of these: boring, dead, clinical, mechanical, objective, studio.B. the warm into: syrupy, colored, subjective, natural, live performance, soulful, human touch.

The choice is: man or machine. Two different roads, both can attain heights or fall into a ditch.
Well I own Atma-sphere MA-1s and think they are among the finest sounding amps in the world. I dont even think about sonic characteristics when listening. I think OTLs are the best tube design provided certain speaker requirements are met.

The thing to ask yourself is why Atma-sphere gets so little attention from the audio press? I will tell you one thing it aint cause they dont sound wonderful. Truly in most cases the actual sound of a product may very low on the list of reviewer criteria. 
It surprises me that people still talk mainly in terms of tonal balance when discussing tube amps.
The warm, rolled off tube sound as it were.
Forget about it! It isn’t what you want anyway. Modern tube sound is better than that.
Tube sound is now about refinement, a sound that never fatigues or is edgy. This is the main reason I love the sound of tube amps. Too many solid state amps try to sound like tube amps by rolling off the highs and detail and do come off as slightly more refined- but boring!
Along with that refinement you get an open, clear sound and excellent transparency.
This is another reason I love the sound of modern tube amps- an open engaging neutral sound.
Last but not least the better tube amps deliver a stunningly spacious 3D sound stage. Solid state cannot touch this.
So kill the idea that the tube sound is about tonal balance, warmth and rolled off highs. That sound gets old in a hurry and it is time to be put out to pasture.  Little wonder solid state amps took off in droves.  
Tube sound is now about high end refinement, lack of any trace of harshness, leans more to a neutral beautiful open, transparent presence and midrange while presenting a spacious wide, deep and holographic sound stage.
The new tube sound should have solid state lovers coming back in droves. 
An n variable Radar chart, Just define your variables and the scales.

Ralph has it right, pick flavors your brain likes.
BTW the REF75 se is a superb amplifier, yes I have heard it length in my system….
Some very good input and some not so much. We build tube components.  Class A tube mono blocks and a hybrid tube amplifier, all point-to-point wiring.  Each has something to offer.  While the Class As are fantastic, the hybrid actually is more 3D sounding - go figure.  So some of the general characteristics mentioned are not so accurate IMO.

You need to focus on a price range and go hear a few.  Then you will get a better handle on what you prefer.

Happy Listening.      
a lot of the main points have been covered here already in the responses to the op’s post

i would add the following...

-- top tube and ss amps converging does not mean they are fully converged to sound the same
-- audio research, for the last 15-20 years at least, has tried to give users their cake and let them eat it too... meaning tubey goodness in holography and tonal accuracy and body, with extended treble and bass depth and impact of good ss...  the ref 75 is an excellent example of them largely succeeding at this endeavor for many applications
-- that said, depending on speaker load, room and volume level desired, how close a ref 75 or 110 comes to say, a top gryphon or pass or hegel in bass response, or how close those ss counterparts come to the arc tube amps in imaging and fleshed out vocals can still be quite variable
@blindjim your description is perfect. Although it may be difficult to put into words I find that when you get to a certain level of performance you are seeking something that is emotionally involving. I remember putting several different SS amps in my system and they all sounded ok. But then I put in a vacuum tube amp and WOW everything just fell into place. It sounded so right in almost every way. For me it wasn't even close. Since then I have tried 3 different tube amps and all sounded better to some degree than the SS amps and 2 were very good. An important note is that the tube amps I demoed were entry level I can only imagine what the "better" units sound like. Amps on the very short list are ARC Ref75SE and QS Mono 120.
@ avanti1960

It surprises me that people still talk mainly in terms of tonal balance when discussing tube amps. The warm, rolled off tube sound as it were. Forget about it! It isn’t what you want anyway. Modern tube sound is better than that. Tube sound is now about refinement, a sound that never fatigues or is edgy.

Couldn’t agree more. Very well said. If folks don’t listen past the tonal balance of a tube amp (which I suspect is usually the first thing most people notice about any system/room), they’re basically missing out on one of the strongest aspects they offer.
@knotscott tonality is the first thing most people hear. But like you and avanti1960 said there is so much more. I notice the decay in the hall or the lingering tone from the instrument itself. cant list it all as there is so much going on. When people ask what I am looking for it would sound like every other audio enthusiast. Yet we all yearn for something just a little bit different. I tell people I'll know when I hear it!
at least to my ears, lately it seems more and more tube power amps tend to sound similar to Ss amps.
That's because you're listening to tube amps that have feedback and probably PP outputs combined with single-ended input circuits. That tends to make them sound 'solid state'. Transistors often sound that way (harsh, bright) due to insufficient feedback, same as what you see in tube amps.
The warm, rolled off tube sound as it were. 
Most tube amps aren't rolled off in the highs unless there is a malfunction. Some are in the bass, especially SETs, since bandwidth is hard to get with SET output transformers.
always I want a tube amp to sound unlike an SS amp… mostly. as the result of distortion? of course not. ultrra euphonic? nope. not that either.
People don't like to hear this, but the primary differences we hear in all amplifiers is due to their distortion signature. People call that the 'sonic signature' but to be clear it really is about distortion. This is simply because the ear converts all forms of distortion into tonality.

The catch here is that the ear pays a lot of attention to tonality that arises due to distortion. There's a tipping point; the ear may actually favor tonality due to distortion over actual frequency response. This is why tube amps with no feedback and arguably more frequency response error due to a higher output impedance might still sound more neutral if distortion is otherwise properly controlled in the circuit.

A peculiarity of the ear that IMO/IME is not well understood appears to be that the presence of the 2nd and 3rd harmonics (in sufficient amounts with respect to the higher orders) can help the ear winnow out soundstage  and detail (there are limits to this; distortion of any type also obscures detail). I'd really like to find out why this is so. As best I can make out since you can't get rid of distortion, what becomes important is to have a benign distortion signature; if that happens it seems that the actual THD the amp has is far less important. That said there does seem to be limits in this regard; if the amp has 10% THD its likely to be less detailed than an amp with only 1% at full power. So the take away appears to be that if your THD is very low, it still has to have the right signature in order for the amp to sound musical.

I dabbled inTubes last year and this year. My conclusion: 

Tube Preamp followed by a GaN FET Amplifier (Orchard Audio Ultra) is my combination of choice.

The Jolida 3502 (KT150’s) worked fine with the BAT VK50-SE but it at times felt like 2nd order harmonics overkill if that’s such a thing. 
My opinion/my ears/my room. Of course your mileage may vary (WTF does that mean anyway) 😂.