I ordered mine this morning as well 
I got the Concert Mod. I need to get a external power source now.
I guess I am a Marantz nut, I have had the PMD430, 660, and now the 661.
My oade concert mod was just delivered and is now sitting on my desk here at work, still in the box. I'll unpack as soon as I get home this afternoon.
I'm also thinking about an external power solution, for longer run times.
Internal rechargeable AA's are abuot 2500mAH, right? so I figure to make it worthwhile, the external pack should be at least 5000mAH... because, is it really worth it to run a 3000mAH external pack when you could get close to that just running the internals? not for me.
I'm thinking about these options:
lead acid:
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=20876V, 7000mAH
$20 (+ charger)
NiMH:
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=50006V, 4500mAH
$30 (+ charger)
the lead acid is 4 times heavier (2.78lbs) and a bigger physical battery, but it does have 55% higher capacity than the NiMH. and its cheaper. and when size is really a concern, I'm planning on running internal batteries anyway.
So those two are my thoughts. Has anyone else put together an external power supply for a PMD-661 yet? Anyone else have other suggestions?