I'm pretty sure the entire Sound Blaster Z-series lineup has the same headphone amp chip used in the Xonar Essence ST(X)...and the FiiO E9, for that matter.
Still, as others have said, I'd recommend getting an external amp if that's what your headphones want. You don't want to replace your sound card every time you need to upgrade amps now, do you?
The SB-Z and SB-Zx, use a different headphone amplifier (cheaper?), then the ST(X) and ZxR and E9/E09K