Allen: All your Dolby answers are here:
Pro Logic vs. Pro Logic II vs. Dolby Digital 5.1
The subwoofer appears to be connected using its speaker-level (or "high-level") input to get its signal. An internal filter then takes out the bass signals for the sub amp to amplify. One advantage of this over a line-level (the "subwoofer out" RCA jack) output is resistance to outside electrical interference.
The difference between Dolby and DTS? Uh oh, watch out what you ask for--you may get it.
Me? I think DTS sounds a little better than Dolby.
runs away............
LJ