how to maximize 2 channel theater sound??

For decor reasons I have decided to keep my music/theater system to 2.1 with two nice bookshelf speakers (Paradigm S1) and a built in sub (in wall Paradigm), all driven by a Bel Canto integrated.

For movies, how much will I gain from getting a pre-processor that can down-mix to 2 channel?? Don't most blu ray discs have a 2 channel track on them anyway??

Also, I understand better players like OPPO BDP-83SE do down-mixing as well. Would this be sufficient??