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!