The Aelius could not drive Vivid Giya G1 and Soundlabs for sure. Put any symphony in and the music wouldn't flow, bass was not good. It just could not handle the swells of orchestral. The Luxman m900u beat it easy on all fronts. The Vitus and Gryphon Mephisto (the latter which I directly compared) are better in different ways but much more expensive. Of those I would take Vitus if someone wants flow, liquidity and tone from SS. Gryphon for grip, bass, jumps, speed, detail, locked in soundstage, for me a bit unnatural. CH and Spectral and Boulder are very good if one wants a clean, fast, transparent sound. However there are so many good amps why lock oneself in a system like Spectral with MIT even if it sounds good? Then you will forever be upgrading from 400 to 500 to 600 trying to get the next trade up to retain used market value. I would go Dagostino over these because of the flexibility.
FM Acoustics is excellent, try to find a 411 used, and 155 pre. Then if you go analog find a phono and you are done.
CAT can drive tough to drive speakers as far as valve push pulls go.
A couple of people have moved from Dagostino and CAT JL7 to Alieno OTL.
From what I have heard of the two brands, I would take a Ypsilon every day of the week and twice on Sunday over Vitus or Luxman. Maybe the Vivid is a tough load but one of hte worst Vivid demos I heard was with big Luxman monos, clean and transparent, yes but completely tone deaf and soulless. Vitus has a rather dark sound IMO where they seem to have deliberately voiced the amps to sound this way (reminds me of the BAT VK 200 SS amp I had). Haven't head the Mephisto but if it sounds like other Gryphon amps then it is lacking in Verve and dynamics.
All your suggestions are not really considering the ultra-revealing nature of the Thiel CS3.7. My partner has this speaker so I have several years experience listening to this speaker regularly with a very wide array of amplifiers. I can tell you that most of your suggestions will not give a satisfying sound as the highs will be revealed to be rather coarse with most of them. It is a quite analytical sounding speaker and needs truly pristine highs from the electronics not to sound relentless.
It is funny because we found that it fell flat with some PP tubes (KR VA880 for example and Octave monos sounded bright and edgy) some SETs (my JJ did not do a good job on it) and low powered SS (Pass XA30.5) and high powered SS (McIntosh MC-501 monos) but rocked with some others like the cheap and cheerful PureSound A30 (even in 18 watt Class A triode mode), VAC 30/30 (awesome bass...really), NAT Symbiosis SE, KR VA350, Cayin 845 SET monos (up to moderate volumes), Wall Audio Opus M50 monos, Line Magnetic monos (can't remember the model but with 845 output tubes), Aries Cerat Diana integrated and Diana Forte.
The FM demos that I have heard at Munich are ok but not inspiring for the kind of money they are asking. The use of the word excellent here is more than a bit of hyperbole...
CAT is about the only recommendation on your list I can get on board with, sorry to say; however, I don't know how you can recommend Luxman or Vitus and CAT in the same post...they sound nothing alike and CAT is far superior in nearly every way to the other two if you really care about realistic sounding music.
Given that we have had very good experience with hybrids on the Thiel, I don't see Ypsilon as a problematic choice and is likely to sound good given how i have heard the highs from their amps (Aelius, Hyperion and Phaeton are the ones I have heard now multiple times and almost always sounding very good...can't say the same for your other choices).
