You simply need a pre-amp with a HT bypass, and an AVR with preamp outputs (or a AV PrePro and an external amp).
I am using an Odyssey Tempest Pre Amp, Outlaw 970 PrePro, and external amps. Works very well. The PreAmp sits between the PrePro and the amps, but only for the Left and Right channels. When in HT mode, the signal simply passes straight thru the PreAmp, such that the volume knob is bypassed (the pre-amp does not even need to be turned on). For 2 channel, simply attach the analog outputs of the source(s) to the back of the PreAmp, bypassing the AVR/PrePro. You can even attach one source, such as a DVD that is used to also play CDs, to both the AVR/PrePro(via coax or fiber optic) and PreAmp (RCA cables).
DVD---> AVR/PrePro---> PreAmp ------> Amps
CD------------------------> PreAmp ------> Amps