I have 2 dvd initial batterys 9v, but was skeptical on using 9v when the voltage problem starts at 8.2v.
how long of run time you guys getting on a 9v dvd?
The specific mics being used could well affect total runtime,
Set up your mics and use the dr680 to record the radio/ambient noise -- see how long it runs before a dvd battery dies and repeat a few more times to confirm. This should give you a good idea of how much time you'll get at the show and you can spot check the recordings for distortion/etc. to address your voltage concerns.
I believe the dvd battery has a voltage regulator and put out close to 9v until the voltage regulator shuts it down.
Like your dr680, my hdp2 uses internal aa batteries. I used a pair of wooden dowels to make "fake" aa batteries, screwed wire leads onto the end of each and use one of my 9v batteries to power it through this connection. I used the other 9v to power it through an adapter plug. This causes my hdp2 to run on the adaptor plug battery until it's drained and then automatically switch over to the dvd battery powering through the aa battery bay. I was able to power my hdp2 continuously throughout all day festivals using this setup.