CMC-8's aka AT933 family are designed by AT to be used in a "3 wire mode". I mean... that's the way AT builds them... 2 wires for signal plus 1 for ground. SP and others connect 2 of the 3 wires together, making them into 2 wire mics, because that way they can make a stereo pair which uses a standards TRS minijack (shared ground between the 2 mics). But then they overload on loud conditions. There is a "low sensitivity mod" which Chris Church created, more for AT853's than 933's, and SP adopted it. Chris has said he doesn't think the 900 series benefit, although others have debated that in multiple threads. Another approach is to use a 3 wire battery box which goes back to using the mics as they were intended. Then you use either mini-xlr's on the ends of the mics, or separate TRS connectors on each mic, into a battery box that's made for the purpose. If you decide to go that route, I'd suggest you get the battery box and reconnection done by the same person, because there is no absolute wiring standard on some of that stuff.
While most recorders wouldn't want to use AGC, if you are using a Rockboxed iRiver, using the AGC in "safety" mode is fine. I always do. What it allows you to do is set the gain up a bit, and it will "ratchet down" a bit as required. If the incoming signal is so hot that it ratchets down below 0 you are in trouble, because once you go much below zero that means you are overloading the input to the iRiver, and distortion will be the result regardless of the meters looking fine.