Because the decoding of DVD-Audio and SACD happens externally to the pre/pro and because that decoding is purely 5.1, the back channels will remain unused. The only way to change this would be to add a layer of signal processing to the 5.1 analog signal path, something that has generally been avoided (which is coincidently why there's typically no bass management on 5.1 analog inputs). The 5.1 and 7.1 surround modes (like DPL II, Cirrus Extra, and the 5.1 and 7.1 stereo music modes) should only be available for digital inputs and in some cases analog stereo (which undergoes an A-D conversion first).

The "back" inputs appear to have been replaced by a switch to enable/disable the 80Hz crossover, by the way.

I could be wrong (happened before!), but I'm pretty sure about this one. They could theoretically send the same signal to surrounds and center surrounds (left surr. and left back, and right surr. and right back), but I doubt that they would.