Chuck,
I've not done it, but any recorder with a SPDIF input should technically be able to record a Dolby digital 5.1 stream. You could also record all six of the decoded 5.1 stream via the analog channels of course.
Hypno,
Not sure I understand your question. You first set the overall input sensitivity for each channel pair to either MIC or LINE with switches on the top of the machine, then adjust input gain as required by the channel trims. If using the V2 before the 650 you can adjust gain on both the V2 and the 680. In that case set the input sensitivity to LINE, then figure out what balance of gain from each device you are happy with.
Details on how to gang trim adjustments (as well as other things like pan and level) are in Tascam's PDF describing new features added with the last firmware update as I recall. I think that is available on Tascam's 680 website. I wouldn't worry about trying to gang channels until you are comfortable with running the recorder. How to do it is covered earlier in the thread somewhere as well, and I'll be happy to explain it again if you need help.