This has come up before, and when it has I've seen comments from Neuromancer (a long-time beta tester for OPPO Digital with some extensive experience with a lot of different manufacturers' players) that consistently said the same thing: players typically can't mix the subwoofer channel of 5.1 tracks into the left/right channels. It is a typical limitation for players with onboard decoding, even before Blu-ray entered the picture. I don't know if OPPO will be able to get the decoder manufacturer to change that. No other players that I know of will do this currently, and I doubt any manufacturers are making an effort to change it.

The 950 is already taken care of for digital inputs: with the sub off and the mains set to large, it will mix the LFE channel into the fronts.