Warning - long message ahead!
I've been in a similar situation recently - planning to buy a Xonar DG as an upgrade to my onboard ALC892.
I ended up biting the bullet as it was very cheap (about 20$, used) and can/could always resell it with minimal loss.
Here is a summary of my personal experience with Xonar DG (and Superlux HD668B headphones), after a week of use:
My expectations: bit more power and headroom, some improvements in separation and soundstaging (not much, but enough to notice and enjoy).
What I can say is that my expectations were only partially met.
The DG delivers way more power (in high gain) than I expected. It can drive the HD668B's to ear damaging levels. In high gain everything above 35-40% in volume becomes too loud on the vast majority of music for more than 5 minutes.
I also invited a friend over to do some more tests with his Fiio E10 dac/amp and Sennheiser HD580 headphones.
The mutual conclusion was that the DG can make the HD580's extremely loud in high gain mode with no sweat.
Unfortunately this is only real praise I can give about the DG.
In terms of sound quality alone I've found it very tough to evaluate.
At the lowest gain setting the output is very similar to my onboard ALC 892. I had a very hard time noticing sound differences between the two.
Sometimes I felt the DG was slightly better with more control and speed, slightly more air, but it was extremely difficult to pinpoint the differences and I wouldn't bet my life on these differences.
The more I A/B-ed the two on the same song the more identical they sounded.
The only encouraging thing is that whenever I felt one of the two sources was (slightly) better, it was always the DG. And never the ALC892.
So, after a week of use and many many comparisons, in my mind, the DG is on a similar level of sound quality with the onboard soundcard. I wish (and ardently hoped) it was better, but unfortunately I could/cannot hear that with my headphones.
With the Sennheisers it's not a contest as the onboard cannot drive them well enough to form a good opinion. From the limited loudness available the conclusions were more or less the same. DG vs. onboard = extremely similar.
That being said, my decision was to keep the Xonar DG.
Why? - Even though it doesn't seem to provide significant improvements in sound, the extra power it's capable of does make a difference.
My main gripe with the onboard was the lack of headroom with some quieter recording (especially orchestral classical music).
Without enough power, the music lacked some of the dynamic contrast and the intensity of the crescendos. And that really diminished the pleasure of listening to these recordings.
With the DG, power is a non-issue and in that regard I can listen to all my music without ever needing more volume.
And the last thing I will mention is about output impedance.
Unfortunately I don't know it and have no way of measuring it. However I suspect it's a bit on the high side (10ohm or more)
Here's why: from what I read some of the signs of high output impedance are boomy, uncontrolled bass and grainy, hard treble. My Superlux headphones are 56 ohm (not terribly low) and 98db SPL.
On some of the music I listen to often and know very intimately I noticed that the bass is a bit more prominent and more boomy than what I'm accustomed to hear from the ALC892.
Moreover my HD668B are known for their rather harsh treble, which I always EQ down in Foobar (about 5db between 5 and 10 Khz).
After a few days with the DG I started reshaping the EQ curve as the treble felt hotter and more 'hard' (if I can say so) than usual.
With my friend's Sennheisers (which are 300 ohm headphones) I didn't notice any of these things compared to his Fiio E10.
The DG wasn't as resolved nor as airy, but overall the tonality and balance were more or less the same. The E10 has very low output impedance (below 1 ohm if I remember correctly).
Now, these are not scientific conclusions and should be taken for what they are - a personal opinion (assumption) based on subjective listening and limited knowledge.
Sorry for the long winded message and thank you to those for those who beared reading it.
I hope my experience would be of some help.
Edited by Jim Wingnuts - 4/4/14 at 6:44pm