id try bracing the guitar in a way that the jack can not move and trying it. if feedback lessens then have the jack repaired/replaced.
Go for a walk. U will be healthier for it and u wont hear the feedback
this is good gear and you sound inexperienced. maybe you're letting the guitar pick up too much output from your amp.<br />
try standing off to the side of the speaker instead of standing directly in front of it.<br />
if the speaker is pointed directly at the guitar, try rotating it 90° away
cheap pickups, guitar pointed at the speaker of the amp which causes the pickups to router the sound throught the pickups over and over causing feeback. Most likely, cheap pickups.
I'd kill for that sound.