Return of The Mac?

Your problem doesn’t seem to be a hardware issue. Check your audio settings line by line. Something is set up to switch automatically with the programming input. I am not familiar with your particular processor so I can’t tell you exactly where to look but I bet the folks at McIntosh can. Good luck. 

The problem seems to be with how your processor is handling 2.0 source materials and whether or not it correctly interprets the 2.0/5.1 input. 

See if you can force it to use Dolby Surround (AKA Dolby ProLogic)

 I just found out this is not an isolated problem. I'm having the same issue with another system in the house. Exact same thing. So tonight I watched HBO and had to turn the volume way up to hear it because it had no center channel at all. It seemed like a stereo broadcast, although the display read Auro 2D processing. Back to the cable company? I've changed cable boxes twice but now wondering if it could have something to do with the cable itself. Could faulty cable installation be doing this?

I have occasionally had problems like this… it seemed to be with the hand shake with the processor on changing audio formats. I don’t remember actually doing anything. The problem just want away… I am thinking some firmware was updated and problem gone.

‘’How long has this been going on? If you have swapped boxes twice, that is probably not it.

I had this problem and it turned out to be a faulty center channel cable.  However it it is more than one system in this case so the common denominator has to be the cable box, but the OP has done a lot on that end.  More information might help.  First, what is processor for the second system?  Second, is it always the same network?  Third, have you tried playing music only in multichannel?

For the heck of it, I would try changing the center channel cables.  Also until the problem is sorted out, change the settings on the processors to two channel stereo, and you should get a more even mix of music and dialogue