Last night was the first time the MixPre-6 let me down. Took it and 8 freshly charged NiMH batteries to a show. After about an hour, it shut down due to low battery. I guess I need to run some tests now to see whether this was a one-off incident or if one of my batteries is dead.
Long story short: better also run a USB battery pack next time as a backup. On the bright side, the show I taped pretty much sucked, so not much is lost.