Hi guys I have a Hartke head that buzzes constantly when the master volume is turned up. I get no signal from the bass coming through at all. The buzz occurs when I turn master volume up, regardless of whether there is an instrument plugged in or not. I've done some troubleshooting, different speaker cabs, leads, instruments, power cables, power sockets, and different locations, and the problem persists. Just wondering if you have any thoughts on what the problem is?