Obie, there is a HDCD indicator that turns on when the 3805 decodes a bit stream that contains HDCD data.
Gonk, I figured HDCD might sound a little better then standard CD since there is an audible difference between DTS 96/24 material and the same material on CD. Maybe the same applied to HDCD.
Since it's easy enough to feed a HDCD signal I figured I'd give it a try since it's basically free. Any player can send the bitstream, the pre amp just needs to recognize and decode it.
Looks like the 990 doesn't support HDCD, not much of a loss considering there were only like 20 titles to pick from anyhow. Good choice to not have to pay extra for support of a non-available format. Still, would have been nice to try it out once.