I recently picked up a Mesa Buster Bass wedge combo. It is a cool amp and my experience with it matches what I have read about it (notably the unusually clean/fast response for an all-tube amp). I got the amp in good shape aside from a replaced screen grid resistor. However, it appears that after a good run with it (30+ mins), the tone thins out and looses warmth. It's not a super warm head to begin with and I might be crazy. For those in the know, does this sound like a possible issue?