Onkyo 705 and DVD-Audio

[BluRayExplosion]

So I'm listening to my DVDA of The Flaming Lips- Yoshimi Battles the Pink Robots (in 5.1!) , and it sounds fantastic, but I'm having some trouble. The receiver doesn't seem to be observing the low frequency cutoff. In other words, the sub is hardly - if at all - engaging, and it's very noticable. I don't think it's just that though, sometimes the bass guitar completely drops out from the mix.

Here's the setup - I'm listening from WAV files ripped from the DVDA on a Windows Vista 64-bit machine with Winamp. My sound card is an Asus Xonar D2X, and I have it connected to the receiver through the multichannel analog (3.5mm) jacks.

I've tried the Multichannel, Pure Audio, Direct, etc. settings, but nothing seems to help. Any ideas?