I remember once a band I was in we had an old Peavey Backstage that was basically on its last leg.

It only sounded good when you went full-tilt boogie on every thing and regardless of the guitar you played the feedback when you were not playing was horrendous.

The only thing that would keep it from feeding back too much was either grounding the strings constantly (with your palm) or each time you stoppped playing you immediately turned doen the volume (and even then sometimes you still had to keep your palm on the strings).