Hi Guys - as you can see from my posts, I'm new to Head-Fi.
I recently bought HD 650's. I trialled both the HD 800's and HD 650's before settling on the 650's mainly due to lack of funds. I currently run my 650s through the Xonar STX.
As mentioned above, the DAC is of high quality, especially for a sound card. Also, as mentioned, the issue therefore becomes that of the amp and whether or not it has enough power to run cans such as the 650's. I can say from personal experience - it has more than enough. So much so, that I prefer the STX over the Lehmann Black Cube that was powering the 650's during my trial. I say more than enough and I mean just that - when at my loudest listening volume, I have media monkey volume control at 70% and the Xonar STX panel control on just 20%. For extended periods, I take it down to 15%.
In no way is the power of the amp (and I know it's hard to fathom as it is a sound card) lacking. Infact, I sometimes worry that maybe I should be keeping the gain setting of the STX panel on 64db - 300db instead of the higher, 300 - 600db setting.
Anyway, I have been reading this forum for a while, thought I would join and put in my two cents... cheers.