A home theater processor would normally take the place of your pre. Usually the home theater pre/pro controls your volume, processes the dolby or dts signal on the dvd and cable/sat box and handles base management - crossover points (also speaker distance and time alignment). All good ones have an analog pass through.
There are some ht pre that don't do any processing, they just let your dvd (or universal player) do the signal processing etc. Mccormack MAP-1 is one that I can think of off hand. There are several more. I don't know what would be gained (other than the tube sound) by keeping your current pre in the chain.
You'll see a lot of advice on this forum saying you don't need a center channel speaker or subwoofer, but to get the real home theater experience I think they are critical. In this months stereophile mag the surround column makes one of the best arguments for a center I've heard. If you like action movies you'll miss a ton without a sub.
No knowing what speakers or amp you are currently using makes its hard to recommend much more.
