My amp just doesn't seem to have the same sound and feel that it had when I first got it, about a year ago now. When I purchased the amp ('66 Bassman) the previous owner had Mesa SRT 425 power tubes in it. The amp hadn't been played in a very long time so I'm really not sure how old they maybe. I've already replaced the preamp tubes not too long ago because one of them had become microphonic. I have experienced noise with the power tubes one time about 3 weeks ago but that may have been due to one of the tubes being loose from being moved around. the reason why I'm wondering if the power tubes maybe dying is because it seems like I have to turn the amp up more to achieve the same overdrive as I was getting when I first got it. The amp has been modded to be more lke the original blackface circuit and the bass channel is now a overdrive channel. The amp is also running at 8ohm instead of 4ohm that the amp likes to see. Is it possible that the tubes maybe burning out or am I just fooling myself?