If you want to know more about how his measurements are done, there are a few articles (iirc one is in video), such as the following:
Article: Understanding Digital Audio Measurements | Audio Science Review (ASR) Forum
He had measured up to 5 channels on some unit but on the power amp outputs only. For preamp/dac of AVRs he always measured the two front channels but had also measure the center channel in one occasion, I might have missed he it was more than once.
Take a look of a couple of examples:
Marantz SR8015 Review (Home Theater AVR) | Audio Science Review (ASR) Forum
Note that Gene has also measured the SR8015 here on AH and you compared the results (AH's and ASR's), they are in general agreement.
5 channel driven meaurement:
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Center channel measurements:
Denon AVR-X3600H AV Receiver Review | Page 14 | Audio Science Review (ASR) Forum
The center channel's SINAD was lower because it was measured with the internal amps connected so at 2 V, the power amp would clip and the higher distortions/noise would polluted the preamp/dac output signal. The front left and right channel in that case were disconnected via the amp assign feature, but that feature can only be used to disconnect the front left and right channel internal power amps.
"As you see, Center channel without amp disconnect suffers a lot.
View attachment 52794