The HD-DVD players will provide a high-bitrate DTS mix of the new formats (DD+ and TrueHD) over optical or coaxial digital. I've heard that it's really quite good.

The 990 offers a 7.1 input, but DVD-Audio is a 5.1 format and the 971H only has a 5.1 output. As a result, there is no surround back data to connect to the 990 - and since the 7.1 Direct input bypasses surround processing, the 5.1 source stays 5.1. I have heard of some folks using splitters to duplicate the side surround data in the rear surrounds (taking the left surround and splitting it to left side and left back surround and doing the same with the right), but I haven't tried it myself.