IIRC, the SU-DH1 uses an earlier implementation of Dolby Headphone (v1). I remember reading it somewhere which was the reason I passed on the SU-DH1. The other reason I passed was because the Victor version was too expensive to import and that the ac adapter was like $40 but you can probably grab a universal at radioi shack for half the price.
I've heard that the Astro Mixamp just utilizes a simplified DSP chip that only has DH2 mode, but consumes less power. No DTS decoding, and probably no Pro Logic II decoding either...things I just might have a use for. On the flip side, it does have microphone mixing for voice chat...except that's not a feature I have much use for on my consoles, especially not the original Xbox I'll likely be using it with. (While I could try out Xlink Kai sooner or later, I just haven't bothered yet.)
Also, I won the Victor version for $86 shipped, apparently with the AC adapter. I think that's a fairly reasonable price to give this thing a chance, given that Mixamps regularly sell for more (though one did sell for just $80 complete here on Head-Fi before).
Edited by NamelessPFG - 12/15/11 at 11:32am