Reason I ask is that my little 7w amp is a master volume only creature that I love. I am led to believe that the benefit of amps like this is that you can get the power amp valve(s) working nicely and get distortion going at the back end as well as the front..... I can't be sure where all the little harmonic fizzes and farts come from in this thing but it's great.
When I first bought it, I lived with it for a bit then swapped out the power amp valve for a posher one. Certainly changed the tone for the better though I can't say if that's because the old one was getting tired (it was at least 3rd hand so I've no idea of the hours it's put in) or because of the snake oil I've breathed in or whether it's genuinely better..... Liked it before, like it better now.
I recently swapped the preamp valve and again a difference but not 'Fender twin to Mesa Boogie rectum frier' change - small, subtle but positive which is what I was hoping for and expecting.
Biggest difference in a given amp (besides the player and guitar used) is what cab you put it through IMO.... Everyone talks of the sound of a particular amp but little hype is given to the speakers used and what they are in.