As far as I was aware, the STX is aimed at the music market, and isn't seen as a gaming card - I certainly bought it for its music qualities, not its gaming ones. (I've never used it set to anything other than 'hi-fi', even when playing the occasional game). :)
I listen to lossless music via pc to hd650's or sr325's - I just want the best sound possible :)
A lot of people purchase them for the Dolby processing built into the card, Dolby Headphone being one of them, and then use an external amp with it because the built-in headphone amp isn't anything to rave about. It's a really great card for multiple reasons, so I guess that's why I'm wonder why you replaced it with a Bifrost. No matter, though. Just understand that using the Bifrost takes the place of the STX and, therefore, you wont have access to any of the STX features such as Dolby Digital Live, Dolby Headphone, Dolby Virtual Speaker, Dolby Pro-Logic II, or the GX Game Audio Engine. Sounds like you weren't using it for any of that before anyway, so no big loss.
Back to your question, there's no point in using the optical out on your STX if you have optical on the motherboard. You're just wasting an expensive sound card on being nothing but an optical port at that point. Plug the Bifrost directly into the optical port or USB port from the main motherboard and you'll be fine. You can also remove the STX completely instead of just letting it sit there and collect dust.
If you are happening to get rid of it, let me know. I'd be interested in picking it up from you if the price is right.
Edited by roguegeek - 2/3/14 at 5:01pm