Ok, that result means that, as far as MC is concerned, it is sending signal to those channels. That suggests the disconnect may be somewhere downstream of MC. Have you verified that those channels play sound correctly with other software? I.e. if you have REW send a signal test directly to those channels on the USB DAC (not through JRiver) do those USB outputs play sound correctly? Can windows play sound from those channels (i.e. try using windows built in sound test functions for 7.1 systems)?
EDIT:
Forget what I said above, the waveform for the two quiet channels looks very odd. Specifically it looks like there's a high pass and a low pass on the same channels at 250Hz? Are you sure your PEQ is still set correctly, it looks like the SL and SR got both a high and low pass at 250Hz, but L, R, RL, and RR got no filter at all (or maybe just a high pass at 40Hz)?