I have a nice 2 channel and a 7.1.4 surround all together in the same room.
The DAC, tuner, pre-amp, amp is the stereo system.
The L & R speaker RCA line out from the surround processor go into Input #3 of the preamp. The other surround processor outputs go directly into the center, surround, rear, and Atmos amps.
When watching a video I switch to Input #3 and turn the volume up to the 12 o'clock position, which matches the other amps. The speaker loudness at 12 o'clock was calibrated using the Anthem room correction software and microphone.
I have to remember to lower the volume before switching back to the DAC or tuner. This system probably won't work with multiple users, a preamp with home theater pass-thru would be a safer solution.