I have has this happen to me twice so far, I would fire it up and there is a loss of power and the bass cuts in and out. But then after 5-10 minutes it is fine with no problems. The first time it happened the amp had not been previously used. The second time, I did a 3 hr gig which it worked fine, then at the second gig (around two hrs between), when I turned it on that issue happened. But as I mentioned, after a couple of songs it worked fine. Could this be a tube issue? The tubes themselves seem to be seated fine in the sockets.