I have a K 100-2 head that just received a new replacement reverb tank from MOD. The reverb works fine but the amp breaks into low frequency howling whenever the reverb is turned up just the slightest bit. It only does then when the head is sitting on top of a speaker cabinet. I understand that this is a form of mechanical coupling and is thus dependent on physical proximity to the speakers, but it seems it shouldn't be doing this under any conditions. Are there any known fixes to alleviate this issue?
Also, I have been doing some background reading on this topic and for reference, I have checked the following things:
1) The replacement tank jacks are internally grounded on both input and output
2) Both RCA jacks on the PC board for the send and return have been cleaned and are nice and shiny