Yuki> Go to the following link for the info you seek
get a passive switch box...run your main outs from the two preamps into the switch box and output it to one stereo amp...I use a DB Audio switch box....very simple and passive..works great with a parasound psp/1500 AV preamp and a McIntosh C22 tube preamp into an Audio Note P2SE amplifier....nice home theatre AND two channel sound with this combo
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.