To be fair, a matrixed rear-center channel is much more than "a blend of the back R and L speakers", and can approach discrete channels in separation. Matrix decoding of a rear channel is the same as the steering with a DPL decoder in 3-channel mode.

In fact, I use exactly that setup to extract a rear channel in my system; a second decoder which I feed the left and right surround channels from my pre/pro, and outputs left, center, and right to separate anp channels.

I am curious: how have you determined you are not getting a rear channel?

Larry
www.fineelectricco.com