2-channel?

Change the speakers for sound improvement or your source equipment, switching the pre-amp will yield subtle improvements if any.

I owned the same processor awhile ago and really like Rotel gear. Unfortunately, the processor is very much the "weak link". A used 1070 Rotel or NAD C162 are both excellent budget pre-amps and would be a big step up. You may also want to consider the Jolida tube pre for about $450..