An external DAC wouldn't be able to send 5.1 even if it had an optical out as it would also need a Dolby encoder as optical (SPDIF) can only carry two channel uncompressed digital audio.
The Creative ZxR would be an extremely poor match for your headphones due to it's high output impedance (38 ohms) and the 50 ohm input impedance of your HD598s. Not all headphones are sensitive to impedance mismatches (AKG K702 for instance) but the HD598s are, their impedance peaks from 50 ohms all the way up to almost 300 ohms around 100Hz. For well controlled bass you should be looking at a damping factor of 8:1 (headphone impedance : Output impedance of the amp) and using them with the ZxR would only be around 1.4 : 1
The HDX has it's own problems, with failing to appear after boot being a biggie. In fact the review sample I had not only failed to appear (after working perfectly for a week) but never appeared again, even though it was in my PC for three months - so I think I can safely assume that it's dead.
The STX is a quality card and built for headphone use. Unlike the HDX it has gain switching and it has a much lower output impedance than the ZxR at 10 ohms. While 10 ohms may still be a little high, the effect will be a lot less noticeable than with the ZxR. My personal choice would be the STX but then I am biased, as I use one for my gaming audio.