Agreed, Byteme's solution works just fine. Just use the L&R front pre-outs from your HT receiver into an unused input on your pre-amp. Note a pre-defined level, like top dead center, and set the pre-amp to that position when calibrating the HT setup.
Send your analog sources directly to the 2-channel preamp and send your digital sources to the HT receiver.
Now, for 2-channel music, your 2-channel rig is totally unaffected by the "lesser" HT setup. When you want to use the HT setup, switch your pre-amp to the "HT" input and set the pre-amps volume to the pre-defined level. You're ready to go.
The only downside in this case for your system is that your tube pre-amp must be powered up and warmed up whenever you listen to music or watch TV using HT. It could be slightly inconvenient and your tube life will be decreased.
Enjoy,
TIC