My Trace 7-band AH250 head is buzzing. I've narrowed it down to the amp or cabs. It starts as soon as i turn the power on. Its buzzing whether or not the mute is on, and wheter or not a bass is plugged in. The buzz does not go away when I start playing, or get worse when I stop. Its just a constant noise. Its not terribly loud, but it used to be silent. It doesn't get worse with more gain. The only variation i can get in the noise is when I turn up the level control on the back of my 2x10 cab..it gets a little wose. Anyone know what this could be?? I replaced speaker cables and intrument cord with brand new ones yesterday. I've unplugged my pedal board, as well as individually tested it with each effect. I've tried 2 different basses, both work fine on my practice amp.