One of the things that escapes many is that things like "frequency response" and "phase linearity" make an underlying assumption that the circuit is, in EE lingo, "linear, time-invariant" -- meaning that the output from two signals at the same time is always the sum of the outputs from each of the inputs taken alone. For most real components and circuits, this is a convenient approximation. In practice, it is a usually "pretty good" approximation, but the cues and clues that our ears and brain use to perceive and localize sound can be very subtle.
Years ago I was working on some of the first "audiophile" CD players. We found that one of the things that dramatically improved the perceived sound quality was stiffer regulation of the power supplies. If you think about it, when you have a "sharp" sound (like a drumstick on a cymbal), you need an instant of high power. If the power supply "droops" a bit, then the "shimmer" of that cymbal might get caught up in the time that the power supply and amplifier are recovering from the hit. It would be very subtle, but that shimmer isn't anywhere as strong as the cymbal hit, and its echo in the hall are even weaker.
Things like "transparency" and "warm" may be the way our brains process some of the low-level differences between the ideal view of an amplifier and what it actually does with complex music.
Tubes are unique things. In contrast to the microns that transistors are measured in, they are millimeters or centimeters in size. They have "grids" or "screens" that have voltage applied to them to change the current that flows through them, and they are called "screens" since they really do look like window screening. "Microphonics" originally referred to when the tube moved or was vibrated by sound, the internal components would move, and would modulate the signal they were passing. Being bigger than transistors, there is a lot more "physics" going on in how they work, so typically a lot more subtle things that they might do to a signal that they pass.
Is "tube sound" better than "solid-state sound?" "What is the best stylus shape?" "Are electrostatic tweeters better than dynamic ones?" "Are hard-dome tweeters better than soft-dome?" "Do Bose 'Direct-Reflecting' speakers sound better than conventional designs?"
Well, maybe not that last one, but the differences between good examples of each come down to very subtle, hard to quantify things. I've ABX-ed some things that my EE training tells me shouldn't make any measurable difference, but my ears tell me otherwise. I've been very surprised, and by things other than the price tag that the item commands.