How to integrate stereo and multi channel?


I need advice. Here’s my problem: I listen to both stereo and 5- and 3-channel music. How can I use the same set of left and right speakers for both stereo and multi ch music?  That is, is there a way to configure the preamps/amps in such a way that I can use only one set of speakers? Note: this is about ONLY music; I don’t do home theater. 
I have a stereo preamp (Pass Labs) , a stereo amp (First Watt), a multi ch preamp and multi ch amp (both Parasound). Plus Tannoy speakers all around. For multi channel music I have a NAD SACD player, and a server hooked up to an Exasound e38 multi ch DAC—both with 5.1 RCA outputs. 

I’m happy to replace the parasound amp and preamp if needed.

I guess I can use a high quality speaker switcher like the Luxman AS 55, but I don’t know what the degradation in signal quality will be.

 Thanks very much. 
ssmaudio
How are you managing two sets of multichannel outputs at the same time, or does the NAD run through the Exasound?
@soix , I have a Parasound preamp that has two sets of 5 channel RCA inputs (plus a bunch of stereo inputs)
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Take the front L/R stereo output from the multichannel preamp (I assume a P7?) and plug that into the HT bypass input on the Pass preamp and critical 2-channel sources (vinyl, etc.) into other inputs on the Pass as well.  When you’re listening in 3- or 5-channel just hit the HT Bypass on the Pass and control volume with the Parasound.  When listening to vinyl, etc. 2-channel sources only the Pass pre is in use.  Obviously the Pass plugs into the First Watt that alone powers the front L/R speakers.  Done.