Following up on my earlier response, I got my new-to-me used 661 last night. I can confirm that a powerrunner powers this thing like a champ. I'd have no reservations whatsoever buying a Tekkeon 3450 for powering the 661.
Doing a run-time test right now. I don't have any internal batteries in the 661, but I've got the powerrunner set at 5V and she's running like a champ right now. I'm testing to see how long it records on a single powerrunner charge and also to verify that my 16GB Toshiba SDHC cards (purchased a few months ago at Sams for $40 each!) won't have any read/write errors when I get to my first show.
I can confirm that the tip you need is the standard size yellow tip connector. I'd be surprised if you didn't already have a couple of these laying around the house. I have at least 4 or 5 of them...a couple of loose tips and a number of wal-warts with this same tip (I'd just cut it off the wal-wart if I didn't have the tip already.)
Not to get off topic, but the Oade preamps i belive send an unbalanced signal and the 661 needs a balanced input, if trying to run the pre>661 analog in, just and FYI, that you probably already knew. I found out the hard way with my 118>661 effort. I actually have the oade WMOD 661, and it has become my small "travel rig", very impressed with it as a box, but i have had some powering issues. I am going to look into the batteries you all talk about here. Thanks!
Richard