HOLY SCHNIEKIES!
$500 for that unit seems just absurd. looks like a great buddy to the ad2k.....
Yeah, I'd go with the 680 at around $500 or so to match with the AD2K. I was lucky as sh** when I found mine on ebay for $375 BIN shipped, which I bought from a professional movie guy who used it for a couple of projects, and it doesn't have a scratch on it.
I do want to comment though about the DR100mkii. The battery life doesn't suck. It's only in a comparison against the Sony M10 that the battery life loses, but then all handheld units lose in in a battery life comp against the M10. In fact, the DR100MKii has a really unique powering scheme (it switches automatically from internal Li-ion to AA or vice versa) that I think is better than almost anything that's out there. I get roughly 7 or 8 hours from a pair of AAs and the internal Li-ion. I think that's pretty good. Most handhelds get around 4 or 5 hours from a pair of AAs. The M10 is awesome and gets something like 20 to 24.
That said, if you want to power mics with phantom without using only the internal batteries, the low voltage cutout is set too high and the unit prematurely cuts out. The cut out seems to be caused by the higher current draw which drags the voltage down, especially on older rechargeable batteries that have a high internal resistance. That is an ongoing problem with that unit...which unfortunately Tascam hasn't fixed by a firmware update even though they did so on the predecessor DR100. Some people are not informed of this problem so they say the battery life sucks, but we've figured out how to get around this premature voltage cutoff if you REALLY want to use this unit to power mics (you need to use higher voltage zinc rechargeable AAs...ni-mh's are lower voltage and they get to the cutout voltage too soon...or of course just use high capacity externals.)
Also I'm not sure where it's been reported that it has compatibility problems with SPDIF, but that's false information. It takes SPDIF fine. It does NOT take AES/EBU.
That said, I had problems with the mini-TRRS 2.5mm connector. As far as I know, I'm so far the only person that I know that had an issue with the connector, but because of my experience, I wouldn't trust it. Others have had fine luck with it so YMMV...though I'm sure most will agree that an RCA connect (such as provided on almost every other smallish dekc) is a more robust connetor thath hasn't shown any issues with looseness.