I have the Superlux 668b and Asus Xonar DG, the SR850 to my knowledge is pretty much the same headphone. I can tell you I can easily justify buying the Xonar DG, but I personally don't think I can justify spending more on the other sound cards with these headphones (which sound great BTW, as I imagine you already know). I can tell you there was a small improvement in sound quality going from my onboard to the DG, not much but it was noticeable, well worth the money. Though part it was from the amp, so I think the DG will be the best bet in comparison to other cards, not to mention you can pick it up cheap.
Looking at the control panel right now I can tell you it has a 5.1 Dolby Headphones and something called 7.1 Virtual Speaker Shifter. But from playing around I will say the 5.1 Dolby Headphones sound significantly better, especially on reference room setting in my opinion.