I've been comparing both my Xonar DG sound card and my Galaxy S2 using my MH1c, B2 and HD 598. I'm not very good at comparing both since whenever I try to remember a certain detail in a piece of music; the other DAC would emulate the same sound, so I would forget how much bass or the specific clarity of a part was. However, when just trying to enjoy the music overall, no matter what, the Xonar DG would always seem to sound simply better in every way for my ears. It would be a bit more tight and warm in bass, less (or more tamed or tolerable) sibilant, more sparkly, and wider in soundstage, yet I personally cannot prove it entirely using minor details.
Whenever I go out and use my S2 and MH1c, I would enjoy my music less than if I had used it on my PC. I had this experience since I first purchased my HD 598; the soundstage and sound overall would be terrible and not as full as with my on-board audio, even at higher volumes. My B2s would feel as they have lost bass impact, as well as have less tolerable sibilance. I don't have any fancy gear, but I tried to level the music to similar volumes so ensure maximum accuracy of what I hear to compare.
I am using foobar2000 and the stock Galaxy S2 music player, flat EQ. Although I have consistently switched back and forth between them each day, I am only going to provide short descriptions of what I have heard using three headphones I own.
Track used to compare: Mezame by Yuki Kajiura (using my MH1c)Warning: Spoiler! (Click to show)
At 1:27, the hiss would be more sharp on my S2, but it is much more tolerable with the Xonar DG, basically it plays and my ears don't absorb the sharp impact of it. In the beginning, the bass would be more warm and flowing, and the highs sparkle a lot more almost borderline sibilant though not close to harsh. If I could explain the signature of the DACs, I'm getting a more bright signature on the S2, while the Xonar DG is more warm. The S2 does not seem to separate the background instruments and bass very well.
Next track: In the End by Linkin Park (using my B2)Warning: Spoiler! (Click to show)
The sound was much easier to separate in this one, probably because the B2s were meant to be for this type of work. The instruments were less separated, and had less detail in the S2 compared to the Xonar DG. The vocals sounded veiled in the S2, yet more forward. There was more bass present using the sound card, and it was less sibilant (and Chester's voice can get sibilant with the wrong headphones). The B2s however create this slight static sound, probably due to some impedance issue.
Next track: Unwanted by Feint (with my HD 598)Warning: Spoiler! (Click to show)
I find it much harder to compare when comparing details, but overall experience, I can say from instinct that the Xonar DG to be more detailed, not as bright, and more balanced. If you skip to 0:39 of the track, I find the bass to have a bit more impact, and a more warm sub-bass that comes right after it. The background instruments are also not as veiled, and are better separated on the sound card. I don't really have much to say after this. The HD 598 just sounds more crisp, lively without being bright, and overall better.
So basically, giving a full listen to music, I enjoy using my Xonar DG/on-board audio much more than using my Galaxy S2. I know the Xonar DG is meant to be better than almost all mobile DACs, but being a rather cheap one, especially one that I cannot differentiate between on-board Realtek ALC892 and the Xonar DG with (without precise comparison of course), I would like to just compare and contrast on-board versus a (lesser) mobile DAC. If anyone has any advice on how I should format comparisons, or which headphones I should be using specifically for these, please post some advice. I'm terrible at writing these. I might try to compare my Xonar DG with the on-board sound to see if there is truly any audible difference between them.
TL;DR: On-board/Xonar DG is less bright, has punchier and tighter bass, more sparkle, toned sibilance, better instrumental separation and clarity, balanced, better soundstage due to better separation, more sensitive powering (B2s had a bit of static), and just overall better.
Bonus: The iPod touch 4G DAC is even worse. Even lesser quality, more veiled sound, probably even more sibilant, grainy to the point it sounds dark, basically it's comparing the Monoprice 8323 (iPod touch) to the Koss Portapro (Galaxy S2).