To use the X-Fi Crystalizer and X-Fi CMSS-3D technology
In that case, things will get ugly. You will need to have 2 connections from the htpc to the amp (HDMI and another for mp3 depending on the X-FI's outputs).
You'll have to switch between inputs on the amp and you'll have to change your sound setting on the htpc each time aswell.
You'll soon get sick of it