Re powering the 660, I've found it's quite conservatively spec'd. I noticed NiMH battery life in the 671 manual was spec'd with 1600 mAH cells. Cell capacity in the 660 manual wasn't specified but I bet it's the same. I'm using 2500 mAH Sanyo HR-3U cells (sold retail as Energizer 2500 mAH at 4/$10 at Target, these are very good cells) and the 660 will record for close to 8 hours nonstop on a charge with these cells, at least with phantom power off. I was originally going to set up an external pack but I find that the 2500 mAH cells last long enough that I don't mind swapping them out once in a while. If I did an external pack anyway, it would be just four NiMH cells (1.25V each) and that should be close enough to 5v to keep the recorder happy. I notice internally the 4 cells are set up in series/parallel for some weird reason, so there's a dc/dc converter in there stepping up the 2.4 volts input.
Here are some cheap 4.8v packs that should also work fine (I don't think the Powerizer cells are of the best quality though):
http://batteryspace.com/index.asp?PageAction=VIEWCATS&Category=471The $12.95, 4800 mah pack should run the unit for way more than 12 hours. Or the 3300 mAH pack including a charger is a pretty good deal.
My pmd660 review mentions a few other possibilities:
http://www.nightsong.com/phr/pmd660.html