our band has got a thirty minute set at a school formal and we're mainly playing rock/indie songs...
only problem being that whilst we were rehearsing some of the more distorted songs on the list we noticed that there wasn't alot of defintion between the instruments and that it pretty muched turned into noise...
any advice?

the lead guitarist is going through a boss gt-10 through the mixer
and the rhythm guitar will be going through a roland cube with a ds-1 out front
Change the set up? Get noise filter equipment? maybe some basic compression sustainer stuff, or just change your setlist. Dunno but good luck out thar!
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roll back your volume pot on you're guitar a few notches. You'll get a slightly cleaner sound but it won't lose the distortion/gain. It'll also add some definition from your other bandmates.
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What band memebers do you have? Im assuming two guitars, drums and bass?

Depending on the size of the bass amp make sure its not on the floor, put it on a chair or something. Thie will help define the bass sound.

Other than that im not really sure. Maybe reduce the amount of distortion on the guitars, if your playing loud you generally dont need a lot.
Use much less gain and less bass+more mids on the guitars. The tone you need for individual instruments in a band situation is quite different to what you might think.
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