I didn't see Lessloss mentioned, maybe I missed it.
In studying the Attraction DAC since I've owned it, we learned that it uses an incredible output stage with single 2134 dual opamp buffering the Phillips 8 pin DAC (I forget which chip) driving output stage which is a pair of quad 4134s! Differential output! There is headroom galore and very high input impedance for the DAC to fully express itself. It is an elegant design that sounds really good. Maybe tat is why many NOS DACs lose the treble shimmer, because the older DACs just can't source that kind of current without immediate buffering, as they were probably designed to be used. The digital reciever is unique to Altmann also. It is a Xylinks (sp?) chip which has his own custom programming. I believe he is upsampling to restore full resolution to high freq signal before D/A. He describes the benefit of high sample rate digital over 44.1kHz on his website, it is very convincing. But I think his DAC picks up the slack for you on redbook tracks. I could be wrong, but that kind of simple math, dedicated DSP is not difficult (for him!!) and since it is a custom design, he can tweak the algorithm and hardware to work perfectly together.
Another DAC that I am growing very curious about is this:
http://www.ecdesigns.nl/He is scaling the mount everest of DAC design, far beyond what is otherwise available commercially. You gotta want it bad to go this kit route. Nice long thread on diyaudio about it.
Rich