Since about October or so, my amp has been making hideous roaring noises when I turn it on, which go away after about ten or fifteen minutes. This happens when I have the amp and guitar volume turned all the way down, on the clean channel, so it's not like I'm failing to let my amp warm up or something. When I play, it usually seems to make this noise when I hit the bass strings, but like I said, it eventually (mostly) goes away. I thought the problem might be due to tubes, so I replaced all four power tubes with new ones back in November, I think. It didn't make noises for about two weeks or so, then it started doing it again. I bought the same type of tubes as were originally in the amp, so I don't know why this is happening. Any ideas on why my amp is acting up, and how I can get it fixed?