I start w/one channel at a time ...
With a test signal, (e.g. pink noise)...
...then run [test again] all together to check interactions.
such as room modes. So far, so good.
Each channel gets independent signals...
...only when watching a movie or listening music
...so if you're checking their interaction using the same [test] signal, that doesn't tell you anything about what it [or they] will sound like in actual use.
Checking "their interaction" (or correctly) the final calibrating, is not supposed to "tell you" what your system will sound like in actual use.
If your system's "interaction" has balanced tone and all channels (2, 5, 6, 7, or 8) are wired w/ matching polarity*, in theory, the system should not "have a sound." It should be "invisible to your ears."
* incorrectly referred to as in-phase
Examine my signature carefully. You will see a very complex system. Tri-amped LCR channels & dedicated surround L&R channel subs not to mention an LFE/sub. I used the same thought process as SteveB to guide me from from a to z. The final steps I use selected tracks from CDs* I've heard thousands of times on many systems over the last 35 years, some costing well up in the 5 figures.
* and certain video passages
Tony