Try Lower volumes and less gain.

Remember that alot of the artists we hear -- though they have "screaming" distorted leads etc. -- that the final "sound" you hear is actually produced during the final mix down and the actual recording was at a lower volume.

By all means turn you r amp to full tilt but keep your volume knob at 0 when not playing and aways try to keep your GTR grounded with a hand on the strings. This should help reduce the feed back.

2 Cents