7.1 surround test, MX121

I have a new to me McIntosh MX121. I am getting sound out of my rears when I do test tones and with when I use the multi channel music mode. However, when I play 7.1 movie content, I get no sound out of the rears.

I’m not sure if this is because for the scenes I’m testing no sound is coded to the rears, or if something is off in the processor or my set-up.

I believe all the YouTube and Spotify Dolby test tracks are actually only in stereo.

Any suggestions on a way to feed a known 7.1 signal from my Apple TV into the processor as a test. Maybe a scene in a particular movie?