I know this has probably been asked millions of times, but I'll make it one more.
I have a bunch of the 9V 5400mAh batteries laying around and a cable that works to connect it to my Marantz PMD-660, which requires only 5V at the adapter plug. When connecting the 9V battery, the PMD fires up OK (of course I took out the AAs to make sure that the power from the 5400 was in fact the battery that was powering the recorder).
So, is there any reason that I wouldn't want to use these batteries to power the 660 for an entire show? (I think I read somewhere that an electronic device may heat up with the higher input voltage?)