I have an Astro A50 installed via 3.5mm/optical output onto a Maximus VI Formula. I have come to the conclusion that since the Mixamp Tx is essentially the sound card/decoder/encoder the on-board audio is somewhat irrelevant.
Supreme FX utilizes DTS Neo PC both Music and Movie modes. Thats not to say that the quality components used on the boards don't improve quality or enhance the experience. Because it definitely does.
I also have a 5.1 Surround Sound/Receiver hooked up to the whole thing via optical/hdmi and the sound quality is noticeably improved over other sources Ive used(usb/g73).
I know Astro recommends to set console preferences to Dolby 5.1/7.1 in their respective settings but my experiences are purely pc related. Ive found that setting it up in either optical or 3.5mm(preferably optical) and letting the Mixamp TX handle the effects is the best method.
Corsair 750D | Maximus VI Formula | 4770K @4.5 | H100i P/P | Vengeance Pro 16GB 2133
ASUS GTX 780 DCII | Intel 520 SSD 120GB x2 | EVGA Platinum 1000W | Windows 8.1