Sonic qualities of SET output tubes?

Blindjim,

Can you readily identify a 300B, 2A3, 805, 211, etc., amp’s sound with your eyes closed most of the time?

You asked this main question and I thought I could give you my opinion on this.

I believe to some small degree,along as each individual different output tube has the proper circuit design and the amp has the correct matching between tube and transformer,the 3 most common lwr pwr tubes,300b,45,2a3 have slightly different sound to my ears and that comes from listening and or owning a few but certainly not all designs.

300b,has the most midrange warmth and body with slight extension roll off on sides of the frequency range.

45 tube has more clarity with less midrange warmth and body,but has greater frequency extension.

2a3 tube to my ears splits the difference between the first two.

Similar sonics to all 3, 
They will have a certain purity and inner glow that also some people refer to as "breath of life qualities"that no other topology of amps tubes or SS have,at least I haven't heard any in 35 yrs.

The big pwr transmitter tubes that I've heard or owned generally sound quite similar with not very noticeable difference in sonics.
 But Offcourse the same as the lwr pwr amps there is just to many variables in the overall designs to really speak accurately to the differing sonics.

Atmasphere provided a great list of variables in design that most certainly effect the sound.

Other Variables that can effect the sound in all tube amps,

Power supply
Type of output transformers 
Types of coupling capacitors 
Different brands of tubes
Different types of driver tubes
Different types of rectifier tubes
Hard wired or circuit board
Solid state or tubed rectified 

And probably other things I'm just not aware of.

I only own one set tube amp which is a Yamamoto A09s which is a 300b 8 watt per channel amp.It's a slightly different design than most 300b amps.It uses Torroidal type transformers in the pwr supply and also the output transformers which gives a slightly different sound.

Way to many variables overall to accurately predict the sound and the best way is always listen to one in your own system and room.

Hopefully I helped you,
Kenny.

@blindjim - the horns are Avantgarde Duos which I have since around 2006- so they aren't one of the newer models but benefitted from some of the running updates prior to the Omega modification. Their internal woofer system is augmented with a pair of 15" subs that crossover very low- e.g. 50hz on a step- 24db/octave slope. With some adjustments to volume and phase (and a little DSP box only on the subs), they give some additional heft and dimension to the Avantgardes without distracting from their coherence. Even that speaker, with its 104 db efficiency, can benefit from a more powerful amp, but there is a synergy with the Lamm ML2, which I like very much. It's instructive to listen to them against the Quad ESL--the Quads are so compromised in certain respects, but their vital strength--the midrange--overcomes most of their shortcomings. 

Most music is in the midrange and to hear it all clearly as was recorded you need full or near full range speakers. Unless you only listen to violin solo perhaps. You also need powerful, life like dynamics. Lamm's SET watts are said to be the most powerful SET watts anywhere, they will drive some dynamic speakers well - you wouldn't have to consider horns only. Give them a listen if you can, if you do and like them a lot - don't call me and Bill names, OK? They are expensive, even used. Vladimir Lamm is good to deal with.
Anyway, you probably need a list of a dozen or so amps to audition. Of course, it's an amp/speakers unit, but still - do you prefer to do speakers first or amp first approach? I myself at this level would try both, in the end I would find the right sound. 

I thought I would just mention tube and hybrid amps that I would want to audition first: LAMM, Ypsilon, VAC, Atma-Sphere, Wavac, Allnic, Jadis, Octave, CAT. Ralph is right - pay attention to the sound not the design. You really need to know what kind of sound you are looking for and the compromises you are prepared to take, and if you already know that - it's both easier and more difficult.
But give horns a listen too if you can unless they are out of the question. In addition to Avant-Garde I would listen to Acapella.
As I said in another thread, $35k for the entire set-up including cables is tough. Yes, I understand you might consider spending more if you see that it's worth it. Some people save $500 on cables after spending $25k on equipment, not wise.

You can tell that this is a great sound, though the base probably doesn't go too low and that it lacks the ultimate authority of much bigger systems. But for a normal medium size rooms it should be just about perfect. However, it's analog source and an excellent one.
https://www.youtube.com/watch?v=zZ8YIow9m2s&t=419s