Pros: surprisingly powerful at high gain, dolby headphone dsp, optical output, low price
Cons: somewhat high output impedance, somewhat high noise floor, equalizer lacks fine control
I've learned a lot about this card over the course of using it for a while, so I figured I would write a review about it. Ok, some strange install issues off the bat - on windows 7 sp1, the initial install was finicky. The first time the driver setup was run, it wouldn't detect the card, but upon a restart, it could. I don't know if this was a microsoft issue or an asus one or possibly an issue specific to me. Another strange thing is that GX was on by default. GX helps with really old games, but it's not very useful nowadays.
Anyway other than that, I actually really liked the asus driver panel. It has a very simple layout. There is one option that is particularly important: the analog out option. When set to headphones, this enables the built in headphone amp. Additionally, it unlocks the various gain options. Low gain is .32 vrms, medium gain was .9 vrms, and high gain was 2 vrms. The output impedance I measured at 13 ohms. I was actually surprised at how powerful it was with high gain. I would say it can power a sennheiser hd600 on high gain, since 2 volts is about what you would want for them, and the 13 ohm output impedance is negligible compared to the 300 ohm headphone impedance. Perhaps somewhat paradoxically, balanced armature iems, and to a lesser extent, the sennheiser hd 558 and hd 598, are not going to work as well with this card due to the output impedance. The hd 558 and 598 get almost a 2db boost at 90hz due to the high output impedance, but people may find this enjoyable though. In terms of noise, I could faintly hear the noise floor of low gain with a pair of shure se215 iems. These are not the most sensitive iems and you generally expect lower noise when there is relatively high output impedance, so this is not a great sign. In speaker mode, which is basically like setting the card to line out, I measured 1.17vrms output with 110 ohm output impedance.
The thing that most separates the asus xonar dgx card from like a usb dac+amp, and probably the main reason to buy one imo, is dolby headphone. If people aren't familiar with dolby headphone, I'd describe it as a super crossfeed. Most crossfeeds work by taking stereo, and moving it in front of you. Giving it that directionality helps reduce listening fatigue. Now, not only does dolby headphone do that better than any crossfeed I've listened to, but dolby headphone also takes the rear channels of a 5.1 signal and moves them behind you. This creates this front-back axis that is just amazing for gaming. When you are setting this up, it is key for the application to be set to output surround sound, and set the asus panel to 6 channel/48khz/headphone+dolby headphone. And since this card has optical output, you can hook up this card to any toslink dac, amp, and headphone combo to turn the headphones into surround sound headphones. So forget crappy gaming headsets; you can get gear with some serious audiophile bona fides. I personally use optical out to a fiio e17 and akg k7xx with the dh1 setting. I should mention that dolby headphone does a lot of mixing, and whenever you do a lot of mixing, there is a risk of clipping. So you should always leave yourself some headroom and keep the windows volume close to 50-60% tops.
Now on to something I don't like about the xonar. Let's talk about the equalizer under the effects tab. An equalizer can be very helpful to ameliorate headphones with known frequency response problems, but this one leaves a lot to be desired. It is only 10 bands, and the change per band isn't immediately clear. If you are going to be using the equalizer, the precise values for each band is stored in C:\Users\User\AppData\Roaming\ASUS\Xonar DGX Audio Center in the cmicnfp.ini file, using multiples of 2^16 to represent change in decibels.
Overall, how is the DGX? Well, these things often go for $20 used, so it is hard to be critical. People who are spoiled with high end dacs and amps might turn their nose up at the output impedance and noise floor of the analog out, but my motherboard, the msi x58 pro-e, actually had a much higher output impedance at 100 ohms, a lower voltage at 1.65v, and a higher noise floor. If that is representative of integrated audio, I would assume the xonar dgx is a pretty solid upgrade over motherboard audio in terms of analog out for headphones. Dolby headphone is awesome, and the spdif out is perfect for using with an external dac. So, all things considered, I really like this card.