Ummm. Some conflicting info there.
Here's how I see it, hopefully Chris can confirm or correct any mis-statments-
The 9100 by itself probably has an optimal signal to noise ratio at it's maximum gain setting, so excluding everyting else, that's were you'd prefer to set it for best performance. Problem is that doing that might produce an output signal so strong as to overload the line-input stage of the DR2d. I'm not sure what the minimum gain available on the 9100 is but if it produces output at it's minimum gain setting (meaning it doesn't mute the output when turned all the way down, some preamps do), your safe bet is to start with the 9100 turned all the way down (not up) and the DR2d turned all the way up (100). If your levels are too low, turn up the gain on the 9100 until the levels are were you want them on the meter. Otherwise, if your levels are too high and are peaking or close to peaking, leave the 9100 at it's minimum setting and turn down the input on the DR2d.
Which way you'll need to go (up on the 9100 or down on the DR2d) depends on the SPL where you are recording and the sensitivity of the mics you are using. Since the CA-14 mics are designed for recording loud music and are not overly sensitive, I suspect you'll be turning the 9100 up, which puts you closer to the ideal signal level for best performance from the preamp. But not overloading anything in the recording chain is far more important, so if you are close to a stack and things are really loud, you may need to reduce the gain on the 9100 as much as you can and still turn things down even more on the DR2d.
Hope that makes sense.