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. 
I used to put the surround sound mains feed through the tape loop of the given stereo preamp, and then set the level on the stereo preamp to a single known spot, and the same for the surround levels out of the connected surround preamp outs on the surround device. 

That way, serious sound is with the tape loop out, and surround happens with the tape loop switched in and that preset and known volume level on the stereo preamp.

I would use my main high end stereo speakers for the surround mains and also for pure stereo, obviously, and then accept the mis-match compromise for the surround, it being lower on the totem pole of importance.

@teo_audio, thanks. I’m trying to understand the configuration. Your turntable connected to the stereo preamp, which went to the stereo amp. And the center and surrounds are handled by the multi chi preamp connected to a multi-ch amp? So in stereo operation playing an LP, the preamp would send the signal to the stereo amp. And of playing multi channel music, you set the stereo amp to “tape out”, turn on tape monitoring and then what? How did the output of your stereo amp connect to the multi ch preamp and how did the multi preamp connect to the multi amp?

BTW, my Pass xp30 has both tape loop and a theater bypass. How is your approach different from using the theater bypass? Sorry for maybe a stupid question.
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.