The only lossless formats on iTrax are WMA and PCM. Given that, the 5.1 mixes are available either audience or stage, but do not contain the stereo mix...so your stereo DAC would be getting a downmix (if you do it via software) or would be getting only the front l/r channels (missing music from surrounds, etc.). My $.02 is this:
* if you want the best stereo download then get the PCM 2.0 (at 24/96...I can't find any stereo at 24/192 even though I thought Mark records some of his DVD-A's at that rez). I wouldn't buy the WMA version cuz that requires a conversion for you (being MAC based).
* if you want the 5.1 mix to burn onto a disc to play via your Oppo, then buy the damn disc instead. That way you will have ALL the versions, and can use my primer here on the Circle to rip the 2 channel yourself. And you won't have the possibly complicated job of taking multichannel files and burning them properly onto a DVD for Oppo playback; instead you'll have the disc.
My $.02.