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??
Yes, that would be more than sufficient.
I may be wrong, but I believe that the down mixed audio on Blu-ray (DVD, too, I think) is lossy Dolby Digital.

Another advantage of the pre/pro over a simple integrated is speaker setup (level matching, delays, bass management) and room mode correction.
Any decent blue ray or dvd player has 2 channel out and internal settings to downmix to pcm output. A processor would not add anything except more cables and duplicated down mixing facilities, which unless you spent a lot wouldn't likely be any better than what in a good blue ray player.
As a complete newbie to home theater I ask- can you use a "sound bar" in conjunctiion with a two channel system? They seem to be fairly easy to work into most design schemes. I realize that they may or may not take another channel or more to function and I don't know if they require the pre pro.