Atmasphere, am I to understand that you're suggesting that as long as an amp avoids actual clipping that it will sound more powerful the more it distorts?
Yes, to a certain degree. It will sound **louder** more specifically.
This distortion thing can be a big deal. To give you an example, we try hard to make sure our amps don't make much in the way of higher ordered harmonics (even orders are canceled not just in the load but in each stage of the amp, and with only one stage of gain, triode operation etc., odd orders are minimized). What we find is that a person that might have a 7 watt SET will turn the volume down before the amp is outright clipping "because it was loud enough', whereas the same customer using our 60-watt amp will drive the amp to nearly full power on the same speaker.
We've actually had customers call up and ask us why the amp does not seem to make any power; that changes if they actually bring a sound pressure meter into the room and see how loud they are playing.
IOW, tube amps make 'loudness cues' that interact with the human ear/brain system in a way that transistor amps do not. IOW its all about distortion.
IMO/IME experience you do indeed want the amp to be as undistorted as possible and lacking colorations (the ear interprets lower ordered harmonics, the 2nd, 3rd and 4th as 'lushness', 'warmth', etc.) because its all about the amp being part of musical reproduction rather than musical generation, if you get my drift. Doing that is a bit of a trick! If you add loop feedback to reduce distortion, you will actually add odd ordered harmonics (to which the ear is quite sensitive, and also finds to be irritating). Norman Crowhurst described this in his writings a good 50 years ago; not much progress has been made in that area since. That is why we avoid feedback when we can. But then you pay a different price for that, see
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php