I thought Xonar could do EAX? (I do not have any experience with the Asus cards btw)

Anyways - I've never had great luck running different soundcards + Sound Blaster in the past. Don't expect to be able to use both of them "one top of the other" for sure - you'll either have the Sound Blaster doing everything and outputting, or the Asus doing everything and outputting. What about the Asus do you like better? Perhaps you could just add an external gadget to the Sound Blaster and accomplish what you want.

Asus sound is very clean and i get what I play. No muffled sound, bass/high frequency add ons, I get what I play. Thats why I like it so much.
But I've always loved Sound Blaster series, and Titanium HD is pretty awesome.

It does EAX, but emulation. I guess its best to simply keep D2X and upgrade to STX II + 6CH when the price tag is right.
I should also try MIDI on D2X.

If you like the Asus, I'd just stick with that - especially if you're using Windows Vista or later, where EAX is emulated no matter what (even on Sound Blaster). Adding the Sound Blaster won't let you have the Sound Blaster do EAX calculations and then somehow feed that into the Asus for output - it won't work. If you need amplifier for headphones or something, it may be cheaper to just buy an external amplifier versus STX II.