https://www.ssguitar.com/index.php?topic=2279.0Peavey Studio Pro 40 with a bad squeel. It seems to be hinged to the reverb section. With everything in it's place, the amp will squeel when the reverb is past 1/2 and the "Post Gain" is past 1/2. Input signal or not.  I was courius if this old post was ever solved but I decided to start a new post.

I'm working on a Crate PA8.  It has a slight distortion through the output section.
I almost didnt aprechiate it at first but as the signal get compicated, you can definitly hear.  Like a slightly blown speaker.  The post you found me at describes the symptom perfectly.