Re: Marshall Amp Help
Pretty much. From this point you'll need to take a look at the following:
Guitar style (semi-hollow vs. solid)
Pickups (low end stock vs. aftermarket replacements)
Proximity to amp (are you right in front of it or 10 feet away)
EQ Settings (you can monkey with these to cut out some of the feedback - an eq pedal in the FX loop works well)
Noise Gate (if your songs have quick stops in them, a noise gate of some kind works wonders)
Hope this stuff helps man.