Any tube can become microphnic, and despite what people will tell you, all preamp tubes can cause feedback when microphonic, even phase splitters. So start swapping out preamp tubes in both, with a known good tube, until you find the culrprit. Also a bad pick up in your guitar can cause this as well. Always best to have multiple rigs and guitars for trouble shooting.
Thanks, yes, I'll start doing some testing on the internal tubes. The guitar I have pretty much ruled out since this issue also happens when the guitar is rolled off. Good idea though - I'll double check the guitar I played last time as well making sure there isn't an issue with the pot or switch on that one.