I'm assuming you've messed around and come up with your own patches rather than going with the factory presets.
There's a little box you can get called a Redbox Cabinetulator.
It's mainly meant for using an effects pedal straight into the board, but it also takes that 'digital edge' off rack based effects. It just goes into the effects send/receive.
You could also get an attenuator so you can crank the amp and lower the volume.
Other than that, I agree with the other guys. You should be able to get a decent distortion naturally from the amp with a decent guitar and pickups.
(although I gotta say that I stopped using Peavey many years ago because I ran into the same problem. Great clean sound, but not really built for heavy rock.)
