OK, I got the new (old model bargain) receiver installed.
Its a Yamaha HTR-5280. The 'five channel stereo'DSP sends
paired stereo infromation to the main speakers,
and exactly the same signals to the rear pair.
The center signal sounds like a R/L mix.
I found this out by trying it, because I still haven't gotten a reply from Yamaha.

So far, very pleased with:
1) Center channel equalizer.
2) Five channel stereo DSP.
3) All 5.1 line level outputs.
4) Video switch.

old..