Apologies, I suspect this is a stupid novice setup question:

I have a new 975 and am having a problem getting it to work correctly. I am running 5.1, with a 975 and a 7125. In surround mode, the dialog is very low in volume and also sounds very tinny and hollow. I have calibrated the speakers with a sound meter. 100% of the dialog is coming from the center channel (If I unplug the center speaker there is no dialog from the L and R). I tried cranking the center channel and reducing all the other channels, and this makes it so that I can hear the dialog, but it still sounds tinny, and everything coming out of the other speakers is too low.
I have tried multiple sources with the same outcome.
If I switch to stereo mode, I can hear the dialog properly, but do not have surrounds.
If I set the center speaker to “none” I get the best sound I have been able to get—I can hear the dialog and the soundtrack, and the rear surrounds work properly, but I give up the center speaker. I tried switching one of the fronts with the center, and the issue persists, so it is not the amp or speaker.
Thoughts?