Annoying 5.1 Sound Issue

It's now possible to download lovely HD DivX files in 960 line resolution with 5.1 sound. The problem is, I don't have 5.1 sound on the Media Centre and have to use 2 channel stereo. I can't see to get the AAC 5.1 codec to handle that without pushing most of the volume into a non-existant sub-woofer.

Any ideas? Or should I upgrade the media centre?