It was the output tubes all along...

@jasonbourne71 1+

However, the S7 has a very flat 4 ohm impedance curve. You probably still have a lot of room for improvement. A good friend of mine has S7s. I am working with him to get the best out of them. I think you should try KT88's and I would match them even though your amp does not require this. All 8 should match! Small differences in amplitude response will affect imaging. Another approach, the one I take is to measure the system by channel at the listening position, but I have the ability to correct any amplitude problem, you do not. The amount of power your amp makes is totally dependant on the power supply. With KT88s in mono configuration your amps put out 200 watts which with subwoofers is more than enough to do the job.  

The swarm is a great system for bookshelf speakers. It is nowhere near powerful enough for for the S7's. You need at least four 12" drivers. Two Magico S subs or two Martin Logan BF 212s would do the job. My friend has four 13" JL Audio subs which I am not crazy about because of reliability issues. I believe the S sub has a complete crossover in it. The BF 212 only has a low pass filter like your swarm. This is not satisfactory particularly with your amps. You need a complete two way crossover with complete bass management. I would put the subs outside the S7s in corners. This only works if you can correct time and phase. I would cross at 80 Hz. What are your program sources? 

Done correctly, your system will be able to play Vini Vici at 100 dB and blur your vision or the Melos String Quartet at 85 dB and make you cry. 

Tube amps are generally not the best choice for any speaker with a nominal impedance of only 4 ohms. Not only is the impedance a problem but also such speakers tend to be relatively inefficient. However it is not correct to say as a general principle that 1 ohm output Z is some sort of cutoff point. Mate that amplifier with an efficient 16 ohm speaker or even an 8 ohm speaker where the impedance at bass frequencies remains 8 ohms or higher, and the match can be superb.

@orthomead Awesome! Very interesting results, and that makes me interested to try some KT170 down the line. I’ve moved on from the 200iQs to a pair of Master 300 monos in the big system (Tannoy Canterbury GR), and Statement 450S (older, non-iQ) in the smaller system (Tannoy Glenair 10).

The Master 300 monos are 100% perfectly sonically balanced for my system & ears w/ KT88 Gold Lions. Phenomenal sound. Hard to imagine any improvement here. I’ve tried KT120 (sixteen !!), and there are things I like, but the old problem of overwhelming bass energy re-emerges, at least a bit. I briefly tried one 300 in stereo mode (w/ KT88), and it was a bit brighter & leaner in this mode - that might come out really nicely with KT120 (or KT150/KT170) in stereo mode! Haven’t tried that yet. The monos w/ KT88 are PERFECT.

The Statement 450S shows a huge change in sound from KT88 to KT120 - more than these other VAC amps (yes, I re-biased). I vastly prefer KT120 here, because the smaller speakers in this system benefit from the extra bass power. This may change when I upgrade & upsize the speakers here (very soon). That said, I heard this very same 450S (w/ KT88) on the massive Acora VRC’s (in a large room), and noticed a lot of the same things as I do on my "crappy" little Glenairs. The KT120’s add a lot of needed meat-on-the-bones. I’m sure the Acoras would’ve sounded much better this way, too. Incidentally, I really like the smaller Acora floor-standers (both of them). If I wasn't such a lifelong Tannoy fanatic it would probably be either Fynes or Acoras for me (I like Magico S series too!). 

On the technical front, these VAC’s at their max rated powers seem to exceed KT88’s specification of 42 Watts (plate dissipation) per tube. I know the power capabilities are driven more by PSU and output transformers, but there may still be some advantage to the extra headroom of larger KT tubes. I seem to like KT88’s relatively less as they go down in number - e.g. a single 200iQ in Stereo mode, at only 1 push-pull pair per channel.

As to the issue of tubes driving 4 ohms speakers - if anything, these VAC amps are optimized for driving 4 ohms like Magicos. They offer taps down to "2-4 ohms", and IIRC they only achieve full rated power into 4 ohms (my 8 ohm Tannoys are out of luck - e.g. the 200iQ monos only hit ~ 140 Watts here). Unless the impedance and phase curve is something completely ridiculous, it’s no problem for a VAC.

@peter_s I purchased my 170s from Vivatubes.com. and had a good experience. 

@mijostyn I would like to try them again.  My initial audition was with the KT-88s and I found them to be somewhat anemic.  When I talked with Kevin about tube options, he was surprised on my take of the KT-88s and opined that perhaps they were past their prime.  I have a very open mind about trying them at some time.  In terms of the Swarm, I use the Dayton SA 1000 amp and bypass the internal crossover.  I spent a lot of time talking with Duke LeJeune about room placement and settings.  Low end isn't a problem in my estimation or others who have heard the system.  I'm sure there is room for improvement though. My crossover is set at 40.  I listen to vinyl predominately.  Nottingham Space Ace with platter upgrade and Van den Hul Crimson Stradivarius. I think this combo outperforms its weight.  Have order in for MSL ultra eminent x.  

@Lewm  I spoke with Kevin at VAC prior to the Magico purchase and he felt it was a good match.  I agree with him.  

@Mulveling.  I would try the KT 170s before the KT-150's.

@orthomead The KT 88s are more crystalline for lack of a better description. With proper subwoofers they will really sing. You have a fine turntable. Do you listen to any digital sources? By proper subwoofers I mean a subwoofer system that can be crossed at no lower than 80 hz and be able to put out a 20 Hz test tone at +10 dB from your loudest listening level  Duke has a fine system, but I am afraid it is a bit lost under S7s. Your VAC and the S7s will be A LOT happier if they do not have to deal with low bass. Separating the low bass out allows you to add the proper amount of gain to approximate the kind of bass you get at a live performance. Do you listen to any organ music? 

A system that is ruler flat in a residential room sounds awful. It will be way too bright and almost bassless. The right curve for this situation that most of us find ourselves in has 20 Hz at +10 dB over to 40 Hz at +10 dB then down to 0 dB at 100 Hz. I like a little 1 dB dip after that to 150 Hz, but that is personal preference. It makes the bass a little drier. From 100 Hz you go flat to 1000 Hz then start a slow taper to 20 kHz where you should be down between 6 and 12 dB. The louder you listen the more pronounced you want this. Every recording has a "right" volume, the volume it sounds best at. Too loud and it gets harsh. Too soft and it will be dull. 

If you have EQ capability you can force your S7s to do this, but the VAC will not like it at all. This is one of the two main reasons subwoofers are important. They are designed to make low bass and you can push them to the right volume without punishing the rest of your system. Even better is using a two way crossover to remove the low bass from the VAC and S7. The system will cleanly attain a realistic amplitude response at a volume that will make you feel as if you are at a live performance and I do not mean ear shattering either. To give you an idea, my speakers will not go as loud as your S7s. They sound bigger, but not louder. I use eight 12" drivers in four enclosures, each enclosure getting 2500 watts. Using fewer drivers would result in more distortion. Your Swarm has four 10" drivers. Two 10s equals one 12. Under a speaker that is capable of more output than mine you are using a subwoofer system 1/4 the size. 

Finances are an issue for most of us, myself included and Rome was not built in a day. You have a great start in the S7s, VAC amp and turntable. The Ultra Eminent is a super fine cartridge and the sweet spot in MSLs range. They are about to release the Signature Diamond for $15,000!  A good subwoofer system that matches your speakers will improve your sound quality eons more than a new cartridge.