So im new to this forum and there might be the answer on here but i dont seem to see the same problem. So my Mt 15 seems be losing power so i thought power tubes. I change power and pre amp tubes and still didnt change. But i seemed to found that if i plug a pedal into the effects loop it seems to be fine and have full power. I started with the one pre amp tube that was for the effects loop then changed them all just now and still no change. Any insight would be great if not eventually i will just take it to my local shop.