Oh oh-hit by the unwanted farty distortion bug last night at the gig (my BX500, not me). Same issue people have reported (and happened to me a few years back).
I hadn't used my BR515N combo in a little while, and decided to bring it last night for the smaller venue with no PA support. For some reason, which turned out to be very good, I also brought along my Ampeg PF500 head as a backup.
I got everyone set up for sound (vocals/kick) and then I plugged in to set my volume to hit for the gig. Was getting that kinda distorted/volume-swing tone...sounded like my battery in my MTD K5 was going. This happened once before with my Carvin and re-seating the tube/ribbon cable fixed it.
I didn't have time to take it apart and do this since the band was almost ready to go...I ran out to the car and grabbed the Ampeg. Used that as a head with the BR515N as a 1x15 cab and was good to go. Sounded great, actually. Love that Ampeg.
Anyway, I will pull the Carvin head this week and see if re-seating the tube/ribbon cable fixes it. The amp has been move around quite a bit, so that could be the issue (I hope).