I think you may be talking about WASAPI?
I don't use my HTPC for music much, but I do use WASAPI for video playback with MediaPortal. This bypasses the windows audio mixer, and sends everything to the amp as PCM. The quality is superb, then I can set my amp to apply a pro logic type setting to stereo soundtracks if I want to, I generally leave 5.1 soundtracks as they are.
As, I say, I don't hardly ever use the music section in MediaPortal, but looking at my music player configuration options, I have BASS engine / ASIO / WASAPI or internal dshow player.
Is this what you might be looking for? It's free software so you could give it a go. This is within MediaPortal 1, there is also MediaPortal 2 but that is very new and I don't know too much about that one, it may have similar music options.