I am wondering a little bit what the problem could be. It is indeed true that the outputs of the 32R in stagebox mode are not equally loud as the mixer's direct outputs, but the difference is marginal (maybe -0.5 dB). There is also a little latency, which you don't hear under normal circumstances.
Assuming that the rack mixer (in Stagebox Mode) is directly connected via a CAT5e/CAT6 network cable, there should be enough volume available on all outputs. All outputs anyway have a Fat Channel and therefore Make Up Gain in the compressor, if there really would be the need to increase volume. The word "power monitors" always implies adjustable inputs and I guess the problem is simply there. Turn inputs and outputs of the monitors up and take any gain reduction away. I believe this will lead to a solution. ;-)