I can help but only a little. I always have run my PMD 661 digi in from my USB pre2. I can say, I have used all kinds of cards, off brands like Emtec (from Microcenter) and Wintec through Amazon and never had any issues. I have used class 10 though, maybe an occasional class 6.
You can preset 3 setups, so you can always do one as line in and one as mic in and see what works best. You can name the presets too, my 3 are digi in , Line, and Mic, but as I said I have really only gone digi in. There is quite a laundry list of items you can tweak in the preset. There is a mic attenuation setting which I think when I tested worked best at -18db, but I cannot be certain. I don't have the deck in front of me, but others can probably chime in with more accurate data.
I think the deck is a bit of a power hog, but I have run everything from a Tekkeon to an eveready low MaH battery and have always easily gotten through an evening. I have not run at a festival though. As I said, it is not the most efficient deck battery wise. I have never run on internals only, but that is the case for me on all my decks always. I just like having the internal batteries be for emergency.
As far as tips, setting the presets will make life really easy. It is as rock solid a deck as you will ever see. The menus are a bit different than most decks but again with the presets, it is pretty easy to just set and forget. I also have a PMD 620 MKII which is just a mini version of the deck, with exactly the same menus and screens but missing the digi in. It is a great little deck. That deck is considerably different than the PMD 620 original model but with the PMD 661 there are not any significant differences between the original and the MKII. I know the MKII has some security features but that is perhaps the only difference.
One last think that is often overlooked is that the Marantz decks do not run on the FAT 32 system which starts a new file after 2GB, so it will record up to 4GB and then seamlessly start a new file.
I know I typed a lot but probably didn't answer all that much. I can say you will enjoy the deck!