It was the output tubes all along...

@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.

When did this shift from rolling output tubes types to digital EQ and sources?

Anyways, I do agree with @mijostyn that a good ADC + DAC conversion process can be extremely transparent. Rather shockingly so. I’ve heard this by playing my vinyl setup through a Meridian 808v5 CD/preamp/DAC (which converts analog inputs to digital). I’ve considered continuing to use it this way for the excellent L/R balance control. Don’t mind the sacrilege. Still vastly prefer many of my vinyl records to their digital versions; it comes down to the mastering.

I’m surprised you find the need for subs at all on S7’s! They’re beasts, and rated down to 20Hz on their own. For music, no more should be needed, unless it’s some massive room? Adding extra energy down there will also add to the challenge of clean vinyl playback. You definitely DON’T want your VAC amps dealing with any subsonic feedback or rumble - it will rob their performance more than extension gains. Have you tried running the S7’s alone lately? Make sure you don't see any woofers flapping during playback. 

Soo. Back to point. @mulveling, since you’ve had extensive experience with the 200iq’s and the VAC master preamp, what would be your next tube to experiment with if you were in my shoes? Also, good luck with Helene-hopefully she powers down some by the time she gets to your neck of the woods.

@orthomead The power tubes are the biggest difference maker, IMO. You’ll also get a bit of different sonic balance if you switch from mono into stereo, but with the Magico’s need for power, you may lose too much there. On my Tannoys, Stereo mode w/ KT120 had really sweet mids and nicely proportioned bass. Stereo mode w/ KT88 was simply too threadbare. I don’t run a sub, which as noted, will obviously change the equation.

For 6SN7 V1 slots, my favorite is the Tung Sol round plate black glass. Hard to find good ones. I’ve had a few that sound GREAT for a bit, but then their heater gives out - usually on just 1 side (I have PTSD from enjoying a "perfect" session and suddenly hearing the music hard pan to 1 side and stay that way - dead channel from a bad heater). Big, lush, romantic sound on those Tung-Sols. Classic tube sound, and then some. Various 1940s and early 50s Sylvanias are also very good. The brown-base 6SN7WGT (w/ chrome tops) is a nice change of pace if you need a slightly cleaner / leaner / brighter sound. The side-getter Sylvania VT231 is always good - overall neutral with a nice, airy sound. I also like the Hytron - big sonic image. Always found the RCA gray glass tubes kina "meh", but they are OK; they don’t do anything particularly wrong but won’t blow your socks off either. Don’t waste your time with later GTA/GTB types in these V1 slots.

For the V2 slots, I find the new Russian Tung-Sol 6SN7GTB gives the best dynamics and punch (i.e. the stock VAC tube). These slots are driving the power tubes and they NEED to be strong. You measure these Russian tubes. and they will generally clobber any vintage / NOS tubes in transconductance readings. These V2 slots also run at higher voltages which the GTB can handle. Older GT series tubes will possibly be running beyond their spec here - but they will work, if you so choose to try it.

For the Master preamp - man, vintage 6922/6DJ8 types are so frustrating. I’ve had a number of them go noisy on me, or other various gremlins (even pins which fall out, broken glass, the works). Even tested tubes from Brent Jesse. I had some Mazda / France 7308 which sounded great but went noisy. Then the various Amperex Holland 7308 are relaxed sounding - probably a bit too relaxed. The 8416 Amperex USA (12DJ8) are similar, maybe a bit better. Think I also like the USA Amperex 7308 a bit better - the Hollands sound good but just aren’t for me.

The Japanese / Mastushita 7DJ8 sound great and have never given me problems - they’re my go-to these days. Good musical tube, good balance. Recommended.

Siemens E88CC are quite reliable and a good choice if you want a less warm tube, but they still have a nice midrange. Funny story, I accidentally ran a pair of Siemens E88CC for ~ 20 hours on the 12V setting instead of 6V. When I finally noticed this mistake they were glowing like the SUN but still played fine. Transconductance measured ~ 20% lower than before this incident, so it took a toll - but they can certainly take a beating! Don’t forget to check this switch position if you’re rolling between 12V and 6V types.

I also have some Philips early 1960’s E88CC which are super duper if you want to wear your sound like a warm lush blanket on a cold day (lol). I like them.

Last year I had moved my 200iQ’s in my 2nd system, and then replaced with the Statement 450S. That ended up being a great upgrade, though I was at first disappointed with KT88’s in it. The Master preamp synergizes at a much higher level with the Master & Statement level amps. With the 200iQ’s, I’m still not sure if I may have preferred the ARC Reference 6 on them.

Fellow loud listener, here lol. I am aware of the OSHA standards :)

Isn’t it really tough for tubes to get along with bass drivers? Isn’t that the real challenge herein?

For the most part, the tubes are fine. If they’re struggling for your power needs, you just throw more parallel push-pull pairs at it (VAC’s Mono mode puts the 2 channels in parallel). The output transformers have the far tougher task, retaining bandwidth at higher power levels. That’s why they’re so big and heavy in VAC’s. A 200iQ is rated 100 Watts x2 and weights over 100 lbs, most of it transformers - it means business, and will outweigh the majority of other brand tubes amps at this power rating. On the extreme other end you had the Carver 275 claiming 75 Watts x2 at just 19 pounds (on par with claims of turning lead into gold).

With Mono use only in mind, what would be your go to tube. BTW, Mike was telling about the VAC 300’s-they sound like they are super sweet. Lots of tubes to roll though!

The KT120’s were still my main choice over KT88 in Mono mode (but it was close). There was a moment in the small system where mono + KT120 was a bit too much bass and KT88 mono was a bit better. But for most configs, KT120 was my favorite. Keeping in mind your system w/ Magicos & subs is quite different from mine!

Now KT120 in a pair of Master 300’s in Mono is again a bit too much bass (stick with KT88's) - but wow, those amps are something else. Highly recommended.