I do love those subs!
The only issue with building something is that I don't have a great way to hear it.. That's a lot of faith to put into a ~$1,000 investment.
Faith that will be
well-rewarded!!
The ZA5.5tt is a 2.5 way design, so it will have full BSC, above average sensitivity. Midbass should not be a problem for such a speake with four 5.25 inch drivers, as long as you build the box to be non resonant and tune the enclosure optimally. This driver is actually well suited to a dual mass loaded transmission line speaker.
There are countless other options, however, when it comes to DIY. You just need to say the word and you'll be building some kick *** speakers.
I have a Velodyne FSR-15 that will be used but I don't like the sound quality in the midbass range.. It plays very low with authority and output, but doesn't stay tight in it's upper range.
I believe the less expensive velodyne subs have pretty high inductance from what i've seen If you want good midbass out of a high excursion driver, you want low inductance. The shivas-X2 for example have multiple shorting rings allowing reponse to be clean up high.
Placement is also a large factor however, and i'm not sure what kind of measured performance you're getting in your room. Your problem might be as simple as subwoofer placement - and multiple subs help address that.
There are superior options to the monitor 11s if you insist on a commercial sub, although they won't match a DIY speaker for the money.
Focal, Infinity, Focal, EMPTek, SVS should all have superior offerings in the price range of the Paradigm Monitor 11s.
Ultimately though, I can't stress enough the importance of room acoustics and placement, when it comes to bass.
