Dan: I already know that the sound of my Alesis MasterLink recorder can be improved with outboard decoding, and as a pro machine it has balanced digital outputs, which currently feed my Theta. Experimentation with the Theta combo years ago showed that a balanced digital connection can offer better sound than S/PDIF (the same also goes for using its balanced analog outputs to feed my balanced preamp), and to me it seems only logical that any DAC with a balanced internal digital architecture and balanced analog outputs also ought to be equipped with a balanced digital input, whether it's a standalone DAC or a disk-player that doubles as one.
Bombaywalla and Russ: Good idea, since Wadia has apparently replaced the 800-series with the similar-looking but SACD-capable 500-series -- maybe there'll be some nice used opportunities with the former. I personally have little interest in SACD, not enough available program material that I want to make it worth my while (same deal with HDCD).
Thanks everyone for the other suggestions. The AA player is a top-loader, with a tubed output stage, both of which I should have said I'm not looking for. The MSB is twice my budget and probably not possible to find used at this time (also the one occassion I brought in another DAC was a Gold Link/P1000, the sound of which I didn't prefer over the older Theta). The Accuphases and Resolution Audio I don't know a lot about, I'll have to research those. I also thought I might like to check out a Meridian, but their affordable players seem not to have digital inputs at all.