I swapped the op amp in my Xonar DS.to an LM4562NA. It sounds very nice indeed.
Result of domino effect add-ons. New GPU, a GTX560Ti, was pretty fat, and overhung the previously occupied (Soundblaster X-Fi, I think) PCI-E* (short 'un) slot, so I had to find a PCI* slotted card. Hence the Xonar DS. The Word was that the 4562 was the almost universally accepted op-amp swap to do.
Hellishly expensive, mind, but I bit the bullet and paid my sick squid, including post & packing.
Sadly, I cannot comment on the SQ difference in the DS, because I was too lazy to install it, try it, then swap it. However, the sound difference between the old Audigy card was definitely appreciable. Less harsh, very warm and rich, it is a noticeable step up, and I have found myself using the headphones a lot more. Music of any kind sounds very good on it
As a further knock on effect, my Hi-Fi owns my Grado SR125s, and the venerable Sony MDR CD470s do computer duty. A lot more often now.
Of course the earpads had split and perished after so many years, and replacements were astronomical / discontinued / unavailable, so I took a punt at suntekstore on EBay £4 including P & P for a pair of approximately the right size and shape from China. They arrived after a few weeks, and are entirely acceptable. As a bonus, I don't walk around with black bits from the scabby old pads on my face any more...
*These may be the other way round...