I have no clue what you know about recording thus far. So with that we will cover some basics.A few more questions my friend.
1) Budget(Most important)
2) How low profile do you want to be.
-You can go miniature electret mics which usually rely on plug in power.
-You can do this with the 661 you have with either putting a preamp or battery box in front
or getting a product from Niant called a pfa. This will take the phantom supplied by the 661
and lower it for the apropriate voltage for your mics.
-If you need to wear the mics you could go with the minis also.
-Again, without a budget this is hard to answer.
Here's a few minis used around here for you to research(we all had to) :
Countryman b3, b6 OMNI| DPA 4060, 4061 OMNIS|Church Audio CA11, CA14 CARDS AND OMNI|
Audio Technica 853 CARDS/SUBCARD/HYPER/OMNI| Nevaton MCE 400 OMNI 3) Stick with phantom mics. A lot more options here with phantom mics.
-These can still be pretty small too, where the capsules are detached from the amp(mic body)
and run back to said amp with a remote cable.
-The possibilities here are truly endless not knowing your budget again. So I will not list them but
PERSONALLY I would skip a stereo mic and get two modular mics.
-Modular meaning the ability to use the same bodies with a variety of caps.
OMNI, SUBCARD, SUPERCARD/HYPERCARD, CARD, FIGURE 8
Again, this is up to you to research what all of those are. They are called polar patterns and are the capsules.
-Try going here to read up on those polar paterns.
http://www.dpamicrophones.com/en/Mic-University.aspx Then go here
http://lossenderosstudio.com/stereo-microphone-techniques.php to read about the
stereo configurations for the two mics.
**Personally, since you are new to this, I would stick with the 661 which you have, get two mics and the Niant PFA
and keep the shit simple. Some disagree but I would also not drop a huge amount of coin on mic UNLESS
you absolutely know you are going to love taping. I have seen a few people drop $ and decide that it isn't what
they thought it would be. Get a solid pair of mics though. No point in starting out with mics that sound like ass.
As pointed out above these would be a great start!
You could buy mTerry's Audix in the yard for $500
http://taperssection.com/index.php?topic=146910.0