To be clear, I was able to match the Xilica processing within a 1/4dB to the EAW-provided transfer functions for the standard, non "focused" KF650z settings. These are not the UX8800 settings. There's no magic tuning here, just a faithful implementation of what EAW intended on standard processing.
To me the two boxes discussed here are in different classes of size/weight, and it really boils down to what you expect out of your top boxes in the low mids when it comes to determining what size of top box to use. Passing on some of that burden to the sub bandpass can be a prudent decision, especially when the subs are immediately adjacent to the top boxes.