THANK YOU for saying this, Rick! I see so many guys rolling power tubes in amps that are driven and voiced by pre-amp gain, and being used in situations where they can’t be played above 3 on the volume knob. I just wonder if it’s worth the trouble...
I would say it depends on the amp, the tube, the player, the playing style, and - often overlooked - the room the amp is sitting in, since when you hear an amp in a room, you're hearing the speaker plus all the reflections off the walls, floor and ceiling. The room itself can augment or interfere with the sound reaching your ears, or it can be neutral, but it shouldn't be discounted as one among other variables.
You probably read my post about the Telefunken 6L6s making a difference in my 100 Watt Lone Star. Heck, you may have posted in that thread?
In any case, the Lone Star is an amp with 5 preamp tubes, and those 5 preamp tubes create much of the amp's tone. Granted, I have NOS GE, RCA and Siemens 12AX7s in the amp, stuff that I think helps make the details in the tone more clear, but the 6L6 tube swap was indeed transformative. It's difficult to describe tone with words, but I felt the overall tone became more lush, with very clear high end, but less harshness at the topmost frequencies.
Of course, I don't know how or why this voodoo worked. All I know is that it worked.
While I also play at higher master volume settings often, this improvement was apparent at very low volume levels. It sounds consistently improved at a variety of levels, actually.
So you just never know. If there's no tone difference in one amp following a tube swap, there still might be a difference in another amp, or with a different playing style, or one of thousands of other variables I can't even think of.
I should mention that I have quite a bit of high quality acoustic treatment in my room, so that might help me hear certain details. I will also remind the reader that I am a complete lunatic when it comes to audio, so there's that.