Hm, maybe I wasn't clear or just didn't understand whay you wrote.

I currently have a 5.1 setup using the 7100 amp. I plan on adding one speaker (unfortunatly don't have room for a 7.1) in the middle of my rear surrounds. So for that one rear middle channel, which output should I use?

I looked at the manual of the 970 and it had it explained for 7.1 but not 6.1.