Some amps sound very different at low volumes and some people just prefer the tone at the higher volumes. Some might say it sounds like shit because it does not sound its best.
At low volumes, I could not bond with the Rectoverb combo I had. It was so fizzy, more so than a 5150, I traded it for a TSL60 (another disappointment) pretty quick. Once it the ROV was turned up to a certain point the fizz went away and it was pure sex, at least for the metal I was into at the time. Too bad at that point the volume was way too loud. I tried 6L6's and EL34's BTW. The Mark V I had was great at any volume since it had all the power switches from 10-90 watts. Now my 6100LM is very similar, with half power and pentode/triode switches I can dial in the sound fairly well. I still think full power is where most amps shine.
Thinking back over all the tube amps I had had, the Peavey JSX and Ultra Plus both sounded great at any volume. The 5150 and ROV were much better off turned up loud.