Should I start the lpf at 400hz and go down from there till it blends right?
More like between 100-200hz. There is expected to be a rather significant overlap with MBM and the mids on your main speakers. I can't really imagine an instance where these would be practical except for some real specific issue worth adding yet another single purpose set of cubes in the room. Then it's likely going to take some rather specialized application of filters in which to manage it all.
This ends up being the long way around to instead choosing the right main speakers for your task, and for the high SPL listener, there is definitely no replacement for displacement. If I was so enamored with the chest slam, I'd set up what I had in more of a near field setup and just get used to listening like that, which I do half the time anyway.
I get mid bass punch even at lower listening levels with the Fusion-12's, and even more so along with a pair of sealed, 12" hi-fi subwoofers.