help needed for new headphone system

Many thanks for these posts.  This information will make my search much easier.  I was unaware of ZMF, and they are only 10 miles from me!  

Lots of great suggestions here. 

I've been wanting to try a tubed headphone amp (most likely Woo), but haven't experienced one, and hesitate to make recommendations for products I have no experience with. 

Your HD650s are really nice and will benefit from a better cable and a good headphone amp.  I have a pair of HD6xx, which are basically the same and replaced the stock cable with a balanced cable and use them with a Sony TA-ZH1ES amp.  I have a dozen headphones including some TOTL cans like Audeze LCD-4, but find myself going back to the HD6xx for their comfort and how easy they are to listen to. 

I have also replaced the stock earpads on most of my headphones.  This doesn't typically result in a big sonic difference, but almost always increases comfort.  

For higher end headphones, the Sony MDR-Z1R are a great value.  Excellent sound quality, deep bass, while being light and comfortable.  

Note that I tend to focus on comfort as much or more than sound quality.  The LCD-4 sound amazing, but they are heavy and not the most comfortable thing to have on your head for a long period of time.

Ditch the HD650. They are ancient tech and dark mud sounding. Go to the nearest high end headphone shop and try as many as possible. 

The new HD800 are nice. ZMF and HiFiman have great cans. Plus more since I quit for speakers. 

Several years ago I started switching to Benchmark DAC3 HGC (DAC/Headphone amp), HPA4 (Preamp/Headphone amp), and AHB2 (power amp). My headphones are Sennhesier HD600 almost every version through HD800s, Gardo GS3000x, Meze Poet, Focal Clear and etc., etc.

No matter what headphones I use, I couldn't be happier with the experience I get using the Benchmark equipment. I have six other DACs to compare with. 

Another headphone amp to look at is the Linear Tube Audio VELO.  It's tube based (6SN7) and uses ZOTL Technology for impedance matching. It also works as a reference level 3 input preamp. Or even a speaker amp for very high efficiency speakers. 

If you’re happy with your DAC and headphones (the biggest improvements in my opinion) then the next best upgrade I ever made was getting the ZMF Aegis headphone amp. It’s not cheap, but it sounds incredible.  And the tube rolling options are pretty much unlimited. Additionally, the community thread on Head-Fi based on this amp is a wealth of knowledge and support. There they have covered a thousand different tube combinations, trouble shooting, equipment synergies, etc.  Plus the designer of the amp participates frequently and is a wealth of knowledge as well. 

I have the Quicksilver Headphone amp and like it a lot.  I use it with Klipsch HP3 and Grado rs1e.   Great amp.  

It was not getting much use, I was on the fence whether to sell my HP gear but zi set it back up recently with a laptop.  I've been using it a lot lately.  I used to use it with a RME dac but unfortunately I sold that.   That was a great pairing.  

I have used the QS with 25 Ohm, 32 Ohm, 300 Ohm, and 600 Ohm cans and it drove them all easily.  

You really do want to try to audition gear in your own home if at all possible. I'm with you in that I don't get much out of listening at audio shows. Too much background noise and too many people around. Find a dealer with a good return policy if you can, or at least with a showroom where you can sit and listen to your own music in a good environment. 

I also agree with the above poster who recommended ZMF headphones. I have the HD600, HD650, and HD800, they are all good in their own ways. I find that I listen to the ZMF Caldera and Atrium Closed much more often than the Senns though. 

Given your tastes I think a tube amp may be your best bet.  Woo is obviously a great choice, and this Quicksilver amp gets great reviews and is considered quite a bargain at $1198 and also well worth a look.  For a DAC I’d look at the Denafrips Ares 15th or the Holo Cyan 2 R2R DACs — both around $1200.  I know with the Ares you can get their Iris DDC and run i2S and get a significant performance boost either now or down the road for not much $$$ (I did this with my DAC and am super happy).  I think you’re on the right track with the Node and LPS (LHY, Teddy Pardo, SBooster).  For cables I and many here have used LavriCables that offer high performance/$ — they just get out of the way and let the electronics do their thing.  For headphones, and again given your tastes, I thought Meze, ZMF, and Audeze would be good choices as they are a little more midrange centric.  Anyway, just some thoughts FWIW, and best of luck. 

https://quicksilveraudio.com/products/headphone-amplifier/

 The ABSOLUTE best "BBC" sound I have ever heard from headphones are those using Bio Cellulose drivers like the ZMF "Atrium & Auteur"..These drivers create a sound that is warm,lush,organic & dripping with ambiance..Be warned though,they take hundreds of hours use to come into true voice & they pair best with neutral source gear..They can become far too dark & obscure fine detail when driven by warm amplifiers & shine with great Class A Solid State like SPL,Denafrips etc...
You should also spend time going through the gear reviews on head-fi.org....

@treepmeyer go to a CANJAM or a shop to demo stuff. Strangers with different hearing abilities and sensibilities will just give you recommends of what they own or like. I'd rather you go listen. Also the Qudelix can be used wired, I have one with my IEM.

And frankly getting an "upgrade" to your Sennheiser will be hard. You're better of testing parametric EQ presets and trying to EQ to your individual pHRTF

@ghdprentice Thanks very much, that it exactly the advice I was looking for.  Woo has a big presence at Axpona but I didn't spend any time with them , unfortunately. I'll do some research on their product line.

@larryi As I posted, I spent time at the last two Axpona conventions listening to headphones.  For me that was simply not helpful .   Thanks for the advice, though.

You need to hear as much as you can of alternatives on the market.  The best way to do that is to go to a major regional audio show.  At these shows, there is a section where headphone people display their wares.  Even if you are not a headphone nut,, this is a fun place to visit at shows.  The people are friendly and they allow you to try different combination of gear, even if that means taking something to another booth/table to try out.

I’ve had as high end a headphone system as I could afford for nearly fifty years. I got serious into digital (sources for my headphone system)  about fifteen years ago. I went through a dozen headphones, and amps, and sources. Given where you are... I would say your headphones are plenty capable. 

The primary determinant in the sound is the headphone amp. Headphones vary in requirements tremendously. But respond really well to massive power. As I said, I have had many head amps. My system got tremendously better when I found Woo headphone amps. Lots of natural musical power. If you start with a really good powerful headphone amp... your headphones are going to sound incredible... then you can add other improvements. I have a Woo WA5... it simply took my system a couple levels beyond where it was... and it was already very good by nearly anyones standards. I would get the best Woo head amp (not WA6... too little power) and a BlueSound streamer. That will improve your system by and order of magnitude. But go all in on a great head amp... I spent thousands on different high end headphones and different amps... one can save a lot of money by one bold move. You can fool around spending lots of money and not get there. 

You can see my headphone system under User ID in virtual systems. I replaced the stock 300B tubes with Takitsuki ($1K each... incredible... but later).