It's just a Plexi cranked up and the guitar in the same room I think.I hear people say that often on this forum with the SV20H. Honestly I have never had the shrieking feedback thing.
It could think it’s due to proximity of amp to guitar or pickups. With my Les Paul Standard (22’ stock) I have never experienced any shrieking feedback. But you’re not the first at all to experience it. Strange one.
I don't get it at lower volumes but when it's cranked I'll get some feedback (the amp is playing at 110dB loud). My office/studio is small. If I turn my guitar volume all the way down it'll go away too.