Re: High pitch sqeal advice needed
What's happening is the volume from the amp is making your pickup vibrate.
Is the Screamin Demon a new factory potted pickup, or is it possible that someone messed with the magnet, breaking the wax seal?
I don't think you mentioned what guitar it's in, but here's some solutions.
Pull the pickup up, and stuff some foam or wadded up paper towels under it, so it's mounted firmer and completely dampened from the backside. Also, make sure the 4 brass bolts on the bottom of the baseplate are snug.
If it's in a pickguard, put several stripes of black electrical tape across the bottom of the pickup.
If those methods don't cure it, take the pickup out, heat up some parafin wax in a pan, and when the wax has cooled a bit, hold onto the lead and submerge the pickup in the liquid wax. Make sure the wax isn't so hot it could melt anything, and don't let the pickup rest on the bottom of the hot pan. Sometimes I put cardboard on the bottom of the pan so the pickup can sit there. Wait till all the little bubbles stop coming up, then put it on some paper plates and throw it in the fridge.
Wipe off all the excess wax, and use an allen wrench to clean the wax out of the hex bolts, and reinstall it. A nicely potted pickup should NEVER squeal, even with the highest gain. Try the other things first, because it might just be the looseness of the pickup mounting, allowing it to resonate and vibrate. It's the same thing that happens when you crank an amp in a room full of acoustic guitars.....they go crazy from the vibration.