Hey man,
I've done this several times over the years. I call it a 2-channel/HT combo system. To do this you will need an HT receiver that has pre-outs, at least for the L&R front speakers. Basically, you run RCA interconnect cables from the pre-outs of the HT receiver to an input on the 2-channel pre-amp. It is helpful if the 2-channel pre-amp has a processor loop/HT bypass. However, if it does not, you can use any unused line-level input, like Aux or tuner.
So, put together the 2 channel system: Source components -> pre-amp -> amp -> front L&R speakers. When you play 2-channel music, you won't even need to power on the HT receiver.
Now, add HT: HT Source components like DVD player, etc. -> HT receiver - HT Receiver L&R front pre-outs to 2-channel pre-amp processor loop/HT bypass or any unused line-level input. Connect the center speaker, surround and subwoofer to the HT receiver.
To calibrate the HT system, turn on the 2-channel rig and the HT receiver. Select the processor loop/HT bypass or the line-level input you used on the 2-channel pre-amp. If you used a line level input, set the 2-channel pre-amp volume to a pre-defined level, such as top dead center (you will always set it to this position when you listen to HT) and calibrate the HT.
So basically your 2-channel system powers the L&R front speakers. When you want to listen to multi-channel HT, you power up the receiver and the 2-channel rig powers the L&R front speakers and the HT receiver powers the center and surround and performs all of the HT processing functions.
I've done this several times over the years. I call it a 2-channel/HT combo system. To do this you will need an HT receiver that has pre-outs, at least for the L&R front speakers. Basically, you run RCA interconnect cables from the pre-outs of the HT receiver to an input on the 2-channel pre-amp. It is helpful if the 2-channel pre-amp has a processor loop/HT bypass. However, if it does not, you can use any unused line-level input, like Aux or tuner.
So, put together the 2 channel system: Source components -> pre-amp -> amp -> front L&R speakers. When you play 2-channel music, you won't even need to power on the HT receiver.
Now, add HT: HT Source components like DVD player, etc. -> HT receiver - HT Receiver L&R front pre-outs to 2-channel pre-amp processor loop/HT bypass or any unused line-level input. Connect the center speaker, surround and subwoofer to the HT receiver.
To calibrate the HT system, turn on the 2-channel rig and the HT receiver. Select the processor loop/HT bypass or the line-level input you used on the 2-channel pre-amp. If you used a line level input, set the 2-channel pre-amp volume to a pre-defined level, such as top dead center (you will always set it to this position when you listen to HT) and calibrate the HT.
So basically your 2-channel system powers the L&R front speakers. When you want to listen to multi-channel HT, you power up the receiver and the 2-channel rig powers the L&R front speakers and the HT receiver powers the center and surround and performs all of the HT processing functions.