A *lot* of preamps these days have a processor loop built in. You could run the LR Front off the preamp, hook the receiver into the processor loop, and run the HT surrounds and CC off the receiver. Then, hook whatever CD/DVD to the preamp. When watching HT, switch in the processor loop. When listening to 2CH, switch the processor out.
Alternatively, if you really want a separate CD/pre-amp setup, you could just use the main out (line level) of the receiver and hook it to an AUX or VIDEO input of the preamp. You would have to match the volumes when you played with HT.