While using a speaker switch will work, there's better ways to accomplish the integration of a stereo system with your theater system. We've done this for many years (and I'm really old :-( ).
Use your McIntosh pre-amp and amplifier to drive your main speakers, no matter what source you listen to.
Connect your sub-woofer using the main speaker terminals, which usually provides better quality sound / integration.
Use an input on the McIntosh pre-amp to feed the analog pre-amp out from your receiver.
This will allow you to use your better quality amplifier independent of what you listen to.
It will also allow your receiver / theater to sound better because it will relieve the receiver of having to power the front main speakers.
The only issue is to confirm your receiver has a configuration which allows the front main signal dedicated to pass out its pre-amp jacks.
Use your McIntosh pre-amp and amplifier to drive your main speakers, no matter what source you listen to.
Connect your sub-woofer using the main speaker terminals, which usually provides better quality sound / integration.
Use an input on the McIntosh pre-amp to feed the analog pre-amp out from your receiver.
This will allow you to use your better quality amplifier independent of what you listen to.
It will also allow your receiver / theater to sound better because it will relieve the receiver of having to power the front main speakers.
The only issue is to confirm your receiver has a configuration which allows the front main signal dedicated to pass out its pre-amp jacks.