I figured out what was causing this but I wanted to relay the story as I learned a lot about troubleshooting in the process (with the help of my bandmates). Couple weeks ago at practice my Avatar 210 started farting out so badly I unplugged it and finished the night with my Avatar 212. I took the 210 home and tested with the same amp (ampeg svt3) but i couldn't get the sound to reproduce. Last week i just ran the 212 and left the 210 at home and the buzzing started again in the 212. I was really upset thinking it was the ampeg (I've already smoked out a MesaBoogie this year!). So we stopped rehearsal and started picking apart the problem included running the poweramp out into a poweramp in on a Fender BXR, no buzz. We ran the speaker out to a PA speaker, no buzz. The buzz occured most when I played a low G, both speakers at almost all volume. Finally we moved the head from off of the cab and the distortion went away. We realized that the vibrations were causing something in the head to loosen up to the point of scewing with the speaker outs. The head was mounted on the bottom of a SKB 6 space case and I've since moved it up and placed rubber washers between the amp ears and the mouting frame on teh SKB. Hoping this will solve the problem. I'm considering laying a foam rubber pad under the head or maybe putting thick rubber feet on the bottom (but the litte feet are pop riveted on and that would be a pain). SO i just wanted to pass that one that there are many ways to troubleshoot and we should trying everything before going on wild goose chases or taking things in to be serviced. I will probably end up replacing the speaking jacks on the ampeg as this is simple and probably prudent.