MM HD-150 buzzes like hell!!

Hi there,
my Music Man HD-150 head has a weird, werid problem: it distorts way to early. even when i got the volume nearly at 0 it distorts in a terrible manner, it more or less just buzzes, there is no clean tone at all. this especially happens at deep frequencies (i play bass) but is not much better and higher ones. in this shape, the head is basically unusable. has anyone any idea what it might be?

You need to figure out whether it's the preamp distorting or the power amp. I assume you have tried changing speaker ca bs so we'll rule that out. Tubes can go belly up and do this, so can the preamp running away and distorting. Bad caps can allow the bass to distort like this so I'm afraid that there is no simple answer. You need to look at all the above or get it to someone who is familiar with MM amp-they are a little different. Mike.