HT Proc/Preamp to Analog Preamp w/o passthru?

I have a surround sound processor/preamp I use for MCH that has balanced outputs to my amps. One amp is used to drive the centers and the surrounds. The other amp is for my main front R/L. The amps are of same family and sonically match allowing a good blend.

I have an analog preamp with balanced outputs that I use for 2CH which has balanced outputs to my main front R/L amp.

Problem: I want to use the same amp for my front speakers for both the surround processor and the analog amp. One obvious way is to simply swap the balanced connections at the amp when I want to switch between 2CH and MCH. Purity yes, but not exactly convenient.

Another option is to run the analog 2CH preamp into the surround sound processor and use the analog bypass but that kinda defeats the purpose of having a pure 2CH path.

The analog preamp has 2 sets of balanced inputs but it does NOT have a home theater passthru mode where the input is fixed. I am thinking use one set of balanced inputs for the 2CH path and the other for the MCH path but again, no passthru mode. Is it possible to run the balanced outputs from the surround processor into one set of the analog preamp's balanced inputs and then simply make sure the volume control on the analog preamp is always set at the same volume? If so, what volume? Would this present any kind of grounding loops or impedance matching problems? It would seem the only drawback would be making sure the analog preamp is set at the same volume level.

Ideas of a more convenient solution? I could sell the analog preamp and purchase one with passthru. :-)

Any help/advice would be much appreciated. You can tell from the post that I'm not the most knowledgeable audio chap but I do enjoy it and learning from these forums.

Thanks!