Hello,
Xonar STX was conceived mainly for the audiophiles, so it really shines in music, being the best consumer audio card (that's why it has only stereo analog outputs; the surround 7.1 support can be achieved only through S/PDIF digital connection).
Xonar Phoebus is an audio card specifically tailored for the gamer's needs, so I would recommend it (Phoebus also has a good headphone amplifier, so it should compensate the Astro 40's missing mixamp).