IIRC there was a sale a while back for the SA5k going for some price around that. However if the SA5k at going for $700 I would just save a bit more and buy a used HD800. They usually go in the 1k-1,1k range however that does assume that you have a good setup already for the HD800.
I hooked my Buffalo III into my rig replacing my D18. It will be somewhat difficult to compare them since I connect to the D18 with a usb/spdif converter and the Buffalo with the same converter. Is there anything out there that can be a usb to spdif for both 2 different DACs?
Does you converter have multiple outputs?