If your connecting the rca outputs of the MP-2 to the XLR inputs of the R44, you will need to increase the gain on the R44 (-8 db would be a good start) to insure that the R44 and MP-2 will clip (or be in the tapers world definition of unity) at the same levels. A typical consumer rca signal is 10-14 db lower than full XLR signal (like that which the R44 can receive).
I'm not sure what you mean by "increase" the gain on the -44?
When I run the Aerco for any kind of rock music, I basically have three options:
10dB gain/R-44 at its default 12 o'clock or maybe down at 10 or 11 o'clock to get not-too-hot levels (I don't recall without looking at it what these gain settings are, but any -44 owners will know what I mean).
20dB gain/R-44 at 9 o'clock or 8 o'clock to get not-too-hot levels
30dB gain pretty much only if it's acoustic or less-amplified stuff. For example, I've recorded a few shows at Joe's Pub in NYC, like The Wandering, where I ran the Aerco up to +30dB into the M10 with the M10 at 4 on the level knob, yielding levels that peaked around -6dB.
I can't imagine adding yet more gain to the R-44 on its end; your levels would go way over. Also, I would think the point of running the MP-2 would be to get gain from the pre and not the -44, so wouldn't the better method be to turn DOWN the -44 until you get decent levels?