As for taking a feed from a sbd, you'll find that more boards have XLR outputs than RCA outputs. And, if it has RCA out then there is a very good chance there are XLR outs unless they are being used for something. With that said, the 1/4" TRS inputs are no different than the XLR inputs other than the shape. AND, the 1/4" TRS input will accept a 1/4" TS input just fine.
AS for battery options, the general consensus:
12volts = best
9volts = OK
Any brand(li-ion, SLA, battery sled) straight 12 volt output = OK
Any brand straight 9 volt (dvd style) output = OK
Tekkeon with voltage sensing = BAD
Anything over 12 volt = BAD
Anything under 9volt = BAD
This is all based on the the fact that there are a number of reported deck failures associated with the Tekkeon batteries. There are some who do not like the way the deck has acted with 9 volts. I personally run 9 volt all the time. And if you run it off of a 12 volt external batttery set up (not Tekkeon) that puts the voltage exactly where the deck wants it to be. These are just "opinions" that were collected form a bunch of us that run the 680. Since most people got away from the tekkeons I have not heard of any recent 680 failures. It could be that Tascam had a bad run of decks and/or made design changes that overcame this problem. But Tascam is not going to tell us one way or the other. If anything their customer support is lacking. The other thing I think most of us do is to run external battery with AA loaded that way if the battery dies or gets unplugged you will not end up trying to recover the data. The 680 MUST be stopped BEFORE the power goes out or it will not save correctly. hope that helps, Kirk