I'm hoping for folks to compare:
well matched amplification and speaker -- with TUBES
vs.
well matched amplification and speaker -- with SOLID STATE
If other factors are responsible for a sonic difference, then I do not have an answer to my question.
This is always tricky! The problem is that many speakers these days are low impedance, a big difference from how it was when tubes were king.
But transistor amps make more distortion into low impedances than they do into higher impedances, and in high end audio the idea is (usually) all about getting the system to sound as real as possible; to that end low impedance speakers really don't have a place in high end audio.
But quite often people compare tubes to transistors on low impedance speakers. Its not an even playing field- if you really want to know what the differences are all about, an 8 ohm speaker (and one that is not 4 ohms in the bass) is how you would do that comparison. My speakers are 16 ohms and I've yet to hear a solid state amp that can play bass as well; if you are playing Sound Labs or Quads you'll probably find the same on them as well (even though a lot of people use solid state on those speakers).
The reason tube amps are still around all these decades on is that they make less audible distortion than transistor amps- the brightness of solid state is caused by distortion (and is as much a coloration as anything tubes do); this is why a solid state amp and a tube amp can measure flat on the test bench but the solid state amp will sound bright and the tube amp will not.