Re: Can preamps be used on a combo?
guitar amp = preamp + power amp + speaker
so your marshall combo has preamp + power amp + speaker. so your marshall doesn't need another preamp but there is no reason why you can't use two preamp in series.
if your marshall combo head has a FX loop, connect the Peavey preamp to the RETURN of the FX loop, not to the guitar input of Marshall combo. In that way, you can bypass the preamp of marshall.
Cheers