I just had a slightly different situation. I decide to purchase a AV receiver to drive my center channel only and use my HT bypass on my preamp. My center channel is driven by the AV receiver and my main speakers are driven by my 2 channel amp. When I'm listening to music I don't turn the AV receiver on.
+1 for ricred! Exactly how my system is configured.
To the OP - you stated in a previous post that you already have pre-outs on your Marantz so your in business. Get a stereo preamp or integrated amp with a Home Theater Bypass feature and you can integrate both your surround system and a high fidelity stereo system into one. Your stereo power or integrated amp will drive your mains full time. No switch boxes - just pure clean HiFi for music as it was meant to be.