The one drawback to digital cable boxes is that the digital coax out is normally just set to stereo output. They don’t broadcast much in 5.1 except for through a fiber optic output, its actually rare to find a digital cable box with a digital coax out. My old box did and I had the same problem, when I contacted the cable company about it (Comcast) they told me that was all it provided and the Dolby Digital logo was on the box but not supported by them. But if I went to an HD Box it was. I have since switched to DirecTV, but that’s another story. Off of a digital cable box, you’re best best is fiber optic if available because most of the digital coax outputs are only set for stereo. You can put your 816 in Dolby Pro Logic II or IIx mode and it will simulate surround sound, but not true surround. You may want to contact your cable company about getting a box with a fiber optic output on it, that could alleviate everything.
Hope that helps.