Thanks for the replies!

Enzo:  Unfortunately, I don't have a scope.  I'm also a bit of a noob, so I would rather not mess with the caps on a 300 Watt power amp.  Death would be very inconvenient right now.  If the bias isn't the problem and the caps look OK, I guess I will probably take the amp to a tech to check this out.

mandu:  I ruled out the speaker.  The buzz is present within the note, but isn't there when there is no signal.

ll81:  Thanks, that sounds like a good avenue to pursue.  It would definitely be nice if this was all just some gunk in the bias pot.

Hi.  I'm hoping somebody here can help me diagnose this.

I have a Mesa Walkabout bass amp (hybrid) which has been giving me a faint buzzy distortion at all volumes, especially on the low notes or when I dig in.  The sound doesn't seem to be affected by turning the volume up or down.  It sounds a bit like the rattle from a loose part on the speaker cab or something, but I have tried it on 3 different cabs and they all had the same fuzziness.  The amp has an effects loop, so I tried running the bass directly into the return and got the same result, so it looks like it's not a preamp problem.

Some history:  A couple years ago I had to replace the power transformer after it melted (because of a faulty speaker cable jack).  Maybe some other component was damaged when this happened?

Thanks!