Hello everyone! I have an amazing Sunn concert controller pa, but it has recently developed an unsettling problem. When first powered on, the amp usually works fine. However, usually after a few minutes of playing through it (with any instrument,) the amp begins to produce a squealing feedback. This issue will occur even at the lowest volumes and continues even if the instrument is unplugged from the amp.

I've messed around with the amp and im fairly positive that the issue is in the preamp. The issue persists on all of the channels which leads me to believe that it is something to do with the master eq section.

Does this sound like anything you've ever heard of before?
Thanks for your time