My Mark V lost its clean sound. For mine it was not a growl, it was more like a fuzzy halo or ghost sound on each note. It was not loud or overbearing and with any dirt you could barely notice, but any clean tones it was there. And it was there in all three power settings. I did all of the diagnostic suggestions Bandit mentions, and recommend them. In my situation the problem remained after all preamp tube tests, so i focused on the power tubes. Since V8 is used in all three power settings, to diagnose, I used the 45 watt setting (which uses only V8 and V9) and swapped, V11 for V8. The clean tone was back in 45 watt, and the fuzzy halo noise was there in 90 watt. To confirm my findings I cycled through all "pairing" options in the inner two 6L6 sockets (V8 and V9). All tubes except the original V8 performed with great cleans in 45 watt setting. I replaced the original V8, and V9 with a new matched set and all has been good since. Recommend that you mark all the tubes and place any good ones back into their original locations. btw, I did the "tap" test and though the problem tube did make a different sound, it was very slight and only identifiable after I knew that tube was the problem.
I know this does not need to be mentioned, but I just have to.... the Mark V is awesome. I have had it for years and am still finding new tones. I have been experimenting with low gain Mark I settings using 10 watts really cranked, with a little reverb - just phenomenal tone, incredibly rich and thick. love it.