Amp / Amp+Dac for HD 650

Depends on your budget, but FWIR the HD650 really should be used with a decent amp otherwise you'd be slighting the headphones quite a bit. There was a time when I was like WTF expensive headphones need amp ?! But once you read a little you come to understand why. They're simply much more powerful pieces of equipment than anything the average joe has typically used before. Does power = quality ? I'd be inclined to say it usually does. Just like one usually gets what one pays for. But as an aside it's not always the case! 

In this case however, whether you get a standalone amp or a DAC/AMP combination unit depends entirely on your budget. As usual. I'd be inclined to say that a DAC/AMP combo unit would be the better option of the two but it will also be the more expensive option. On the other hand I imagine the DAC on your Xonar won't be bad at all so if you feel like saving money and instead getting a decent standalone amp within your budget then you can obviously do that too and above all, be happy with what you have.

Nonetheless I imagine a decent standalone DAC/AMP unit would go such a long way, and you gain portability. The downside is it costs a decent amount of money, but then again most things worthwhile electronics in life often do. Why not give audio equal financial backing ? After all it's the most important factor in immersion. Which in turn is the most important factor in entertainment. That's the reasoning I give it at least.

You can find a TON of info on this question if you search the forums a bit. The HD 650 is a very popular set of headphones and there are lots of threads on budget setups.

One thing to keep in mind is that you can find some really great used gear here on the for sale forums. Your price limit will go a lot farther if you buy used.

Welcome to Head-Fi, and sorry about your wallet...