I know that one Realtek card is also really good (just don't know the type). Just an on-board chip but some motherboards don't have a HPF built-in which also makes it DC coupled.
True to all that. Even my crappy Acer laptop can do a decent job in MPX encoding (it needs calibration, though). Look at the realtek alc thread, I've put some links there. As for Asus Xonar itself, well... I'd be very reserved in using one for the MPX. It's just not for that. The one I had back in the years had some strange equalization always turned on via driver and I couldn't disable it no matter what I did.