Like others have mention, yes, this will give you Dolby Headphone over optical, but if you're going to use a Modi as you DAC and Magni/Vali as the amp, then there is no reason to blow money on a STX. Just get the much cheaper Asus U3 or DGX as you are only using the soundcard for the virtual surround processing, both will do Dolby Headphone over Optical out.
Be careful of this one. Not all soundcards support WASAPI properly or they only work over certain output ports/jacks. My X-Fi Titanium (non-HD) will freeze my Win7 box if I try to play music with a WASAPI driver over the SPDIF port (foobar2000 & MediaMonkey). It will however work if I'm just using the standard headphone jack of the X-Fi (but the headphone jack is too weak for my AKG K702.65 and I don't want to double-amp). Some other Creative cards will work with WASAPI over SPDIF but you will need to do your homework on which ones do and don't.
Interesting, When I set foobar to use WASAPI it runs fine over s/pdif even on realtek onboard