This is a good point which I hadn't really thought about until I watched the video below. In the video which concerns a hot amp he switches a Chinese tube with a GE tube before changing the bias to correct the over heating. One tube dissipates 19.6 watts and the other 16.7 watts that difference should account for a change in sound that doesn't correlate with the brand of tube. To really tube roll I think you have to re-bias for each tube so that the plate dissipation is the same for each tube. My apologies to the OP for hijacking an old thread. I enjoyed the video very much.