I put my cabinet on 4 (which is pretty high I suppose) in my room which is rather small which could be the reason why, but I don't use much distortion at all when I play. If I lower it at all, it becomes crunch rather than distortion and if I raise it, it gets much worse. I think my gear or dialing might have to do with it and since I'm new to guitar I'll post the settings I use just incase it is. I currently use:
Crate GT1200H 120W Guitar Head
Marshall M412A 300W Guitar Cabinet
BOSS BC-2 Combo Drive Pedal
Various guitars such as:
Ibanez Stagestar with a Crunchy Pat pickup in B Flat
LTD M-17 7 String with ESP Designer pickups in Drop G
Maestro Les Paul stock PUP in Drop C

My settings:
Low: 7/10
Mid: 3/10
High: 6/10
Reverb: 2/10

Distortion Pedal:
Volume: Half
Bass: 2 O'clock
Treble: 2 O'clock
Sound (Crunch): Noon

For some reason if I lower it at all, it just becomes a clean boost and if I raise it, the feedback becomes even worse. Any advice would help!