a system that splits the subwoofer into a ported um18 mini-marty type sub (large 17hz tune ported) and a pair of sealed pa460 subs (4 cubic feet) can be made to combine to produce the combination of the two (the red line).
in this system the blue line (mbm's) will be carrying most all of the load from 70hz up, will be sharing the load with the ported cab down to about 40hz and then the ported cab will take over from there down.
with the um18 in a ported cab having to cover the midbass by itself, the driver is consuming lots of amp power and lots of current through the driver. this can cause some power compression and some other artifacts that may be why you aren't completely happy.
the red line is how much power the amp requires to produce the output of the red frequency response curve using the um18 driver only. in the combo system, the um18 power requirement falls to the green line and the much more efficient pa460 take over the upper end (the blue line).
with the appropriate eq, phase between the ported cab and the sealed mbm tracks nicely.
eq on the system to start would be simply the 17hz high pass filter on the sub (as would be done anyways).
then a 50hz low pass 4th order LR 24db/oct would be set in the inuke on the um18.
the pa460 sealed cabs would get two filters.
a 3rd order high pass butterworth (18db/oct) at 40hz to integrate the spl with the um18 around the crossover region
then there would also be a 4th order high pass butterworth (24db/oct) filter at 20hz. this one doesn't impact the frequency response, but pulls the phase into near perfect alignment.
with the system all integrated, any overall eq that you'd like to apply to the result would simply be added to both the sub systems. so if you wanted +3db at 30hz q=0.7 added to the red line, that filter would be added to both the um18 channel AND the pa460 channel.
any low pass filtering bass management from the avr would be applied to both input signals before being process in the inuke dsp.
with your smallish room, that modeled red line would probably end up pretty flat (or even rising a bit) down into the teens.