Amp is borked. What do?

Hey guys. I have an Ampeg Micro CL with the 2x10 cab. Suddenly my 0db input makes a terrible squealing noise when I plug in a cable. Sometimes it's quiet until I touch the tip of the cable or plug it into a bass. Then horrible squeal.

The -15dB input seems okay though. I can plug into it and it's fine, but swap to the 0dB input and it wails like a banshee. It's not like feedback, it's super high pitched right away.

Different basses, different cables, direct in or using the effects loop, active and passive instruments. I think I've eliminated everything but that input jack or it's circuit.

Any ideas?