Someone already mentioned the Isabellina, but it's too expensive and I'm looking for something that's marginally portable. Scroll down to my second post in this thread for info about how I'm going to use the DAC. I can't very well carry that thing to campus on my skateboard!
Some1x, the Wavelength looks great, and I might contact Gordon, but it's a bit more than I wanted to pay for just a DAC so if I bought one of those new I'd probably be using the built-in amp as well. If the Proton were to be run from an optical input, however, it would require more modification than just the addition of an optical input. The internal pot is controlled by a volume slider on the desktop; there's no knob to control it. Also, while the DAC itself is powered by the internal lithium battery pack, a variety of its features are bus powered. So a Proton that was usable with the Zune would truly be a custom product. Since the Proton already costs more than I want to spend, this probably wouldn't be feasible for me. This makes me wonder whether or not I wouldn't be better off anyway with something along the lines of the soon-to-be-released Archos 9, which is a small slate PC with a 9" screen that runs a light version of Windows 7 and may be able to host this DAC. It's looking like my setup might turn out to be so big that I might as well just do this. It sort of seems like a waste of the JH13's portability.