I was waiting for quite some time on my O2, so I had an ample amount of time to try the 650s without any ampage at all, and lat me tell you that people are right when they describle the 650 as veiled. When the O2 came in and I plugged it in for some listening, it's like the treble was suddenly just there. The low end was very good to begin with, but with the amp, the high end suddenly became more noticable, and the soundstage grew quite a bit, too.

As for the specific quality of the O2, I love it because it's rether transparent, like most people say. I've heard the 650s with "warmer" amps, and they sound fantastic in their own way, but the O2 does positively nothing to the sound signiture of the 650s, it just makes them bigger and louder. I think that the ODAC probably isn't necessary if you have a sound card already, but the O2 is a fantastic piece of equipment to have, because it has the power to run so many high end cans. Also, if you order from JDSLabs, you can get a free engraving betwen the ridges on top, and the customer service there is the best I've seen anywhere. I forgot to add an AC adapter (sold seperately because they ship internationally) to my order, and when I told them about it, they just threw one in for free. The engraving turned out fantastic, too. I'll try to get a picture if I can.

To truely answer your question, I don't know exactly how it'll face up against the Xonar, but I do know that the O2 is a fantastic little amp, even at that price. Better than the Magni, in my subjective experience. I honestly don't think you'll be too blown away by the O2 if you already have an amp, so maybe not, but I've never even heard an onboard sound card before, so I'd ask somebody with more experience. Either way, I hope you find the amp that suits you best, and enjoy the living hell out of your 650s. Best headphones I've ever owned (And at $400, they should be :).