My band has a practice room that has cement flooring with very thin carpet and it is plain drywall. It is about 15 feet by 10 feet in size. The acoustics make everything pretty loud. When we practice, my Dual Terror and PPC412 is usually at around 9 - 10 o'clock on the volume and the gain is a little more than 1 o'clock. For maybe the first 15 minutes the sound is perfect but then I start to notice I lose the bottom end slightly and the gain/sustain is noticeably lost. When playing clean it just starts to sound thin and a little weak. Something just doesn't feel right. I replaced all the power tubes and I have swapped out the preamp tubes with some vintage 12ax7 from another amp. Could the problem be that I just need to get new preamp tubes? Possibly just some ear fatigue messing with my head? Thanks.