I think your comment has merit about the ULS-15 mk2 with the VTF-3 HO. The response might create a "bottleneck" from the larger driver to the smaller driver.
In terms of your MBM, you could probably use another sub from another manufacturer. Depending on your budget, you can probably these products from Monoprice https://www.monoprice.com/product?c_...seq=1&format=2
or this https://www.monoprice.com/product?p_id=9723
. These play down to only 50Hz.
You can even use this sub from Parts Express https://www.parts-express.com/dayton...oofer--300-627
. This plays down to 40Hz.
If you have a bigger budget, you can use this as well http://www.rythmikaudio.com/FM8.html
. This can be used as a MBM with a cutoff at 50Hz.
With any of the above mentioned subs, you can connect these and your VTF-3 HO via a Y-splitter cable from your AVR. Since these subs can play down to 40-50Hz (place them "near field" for the mid-bass "punch") and set your VTF-3 HO crossover switch to "in" and set the crossover knob to 40 or 50Hz (depending on which sub you use as your MBM). From here, you can set your AVR crossover for your whole system to the lowest frequency range of your speakers. Preferably 80Hz to let the MBM do its job. This allows your VTF-3 HO to handle all the rumble impact below 50Hz.
I hope this has helped in any way.