I built 31 hz pa-460 VBSS's (John's ported versions from page 27 of the thread) with the intention of integrating them in with existing um18-22 mini-marty's. The VBSS build came out perfectly and they graphed exactly as they should. However, I found integration exceedingly difficult with the different port tunes (18 hz mini-marty's and 31 hz VBSS's). There was no combination of timing, placement, phase, or slope that that didn't result in destruction at some point in the bass spectrum. I was able to do a work-around by splitting the bass spectrum with the marty's doing under 40 Hz and the VBSS's doing 40-80 or 90, but the end result was not worth the addition of more amps and subs. Room issues come into play and the room dictates where you need to place a sub to make mid bass and where you need to place a sub to make low bass. In my case my room constraints ended up being an issue and I ended up taking the VBSS's out of the mix.
I hear tale of folks finding some combination of phase/timing/slope that allows them to integrate everything while running full range, but I wasn't able to find one. If your intention is to run the mini marty full range with the VBSS you may have a very hard time integrating a 31 hz box. If your intention is to split the sub signal and have your mini marty take the low end, cross at 40ish, and have your VBSS take the upper end, it may or may not provide some benefit depending on room and placement. Just some food for thought before you start sawing and sanding.
SPEAKERS: 3 x DIYSG 1099's LCR; DIYSG HTM 6's and 8's surround; 4 x RSL C34E's atmos.
SUBS: 4 x UM18-22 Mini Marty's and 2 x 21DS114-4 Devastators
SIGNAL CHAIN: Denon X4400; Minidsp 2x4 HD; 2 x Jensen Iso Sub 2RX; 3 x NX6000d.