I run a 670, you may remember me from DBT in C'ville. I started having issues with some internal NiMH Energizers earlier this year, and since some were older than others, I just replaced them all with brand new Duracell 2650mAh AAs and everything has been great since. Full disclosure: I am only taking a digital input and that lessens the drain, but for Trey last weekend, I ran 3+ hours with the display light on about half the time, and it was showing a full charge at the end of the show. Next night, ran 2:50 hours straight through for Dave & Tim with the display light on the entire time, full charge showing at the end.
I hear it's very important with the 670/671 to have the menu setting specified for NiMH versus Akaline for internals, but I don't know what specific setting it should be for an external supply, if any. I am going to offer a suggestion that you invest in a new name brand set of 8 NiMH AAs at 2500mAh or more, and put them in freshly charged before each show. Then, even if your external battery croaks, the internals will pick up the slack and keep you rolling seemlessly.