Subwoofer damping

a 12 db slope from say 50 should start from 38 db for LF???

Crossover Frequency 50Hz, 80Hz, 110Hz high pass, 50Hz-140Hz adjustable low-pass filter
Crossover Slopes 12dB/octave high pass, 12dB/octave low pass
Phase 0/180 degrees switch, 0-90 degrees variable
Input Connectors XLR, RCA
Output Connectors XLR, RCA
Dimensions 2.125"H x 17"W x 11.5"D
Weight 9 lbs
Controls Gain, LFE Gain, Phase, LP, HP, Boundary (+/-6dB), Stereo/Mono, Trigger mode

I meant...a 12 db slope from say 50 should start from 38hz for LF???

Crossover Frequency 50Hz, 80Hz, 110Hz high pass

With a 12 db slope this will be down 12 db at 25 HZ and down 24 db at 12.5 Hz...

As I mentioned, this filter look like it was designed for the feed to main speakers.

50Hz-140Hz adjustable low-pass filter

This filter appears to be the feed for your subwoofer and it does not appear to have protection or a filter for ultra LF (which is normal because a modern subwoofer usually can handle ultra LF often having momentary gain reduction built in etc.)

If you look at the behringer device then you will see it has a limiter to protect bass woofers...this could be useful.

I am not familiar with NHT x2 - sorry - I just thought you might need pro bass management seeing as what you have was previously in a disco bar,,,

The box is still abnormally huge for a typical driver of the size -- i.e. two drivers in a shared 14 cu. ft. enclosure is theoretically the same as two drivers, each in their separate 7 cu. ft. enclosure. If the port lengths/sizes are different for each woofer enclosure, and/or the volume of each enclosure is substantially different, then the woofers/enclosures are tuned differently. But from your description of this as a "stereo" sub, that's probably not the case, and your stuffing difference is probably arbitrary.

I would recommend that you figure out the exact model of the drivers in your box, and download a software speaker-box simulator like WinISD. You can then use the software to get an idea about the effect of the size/tuning of the box versus the driver, and how they interact . . . WinISD Pro also has some rudimentary tools for predicting maximum cone excursion and stuffing losses, so you can study these effects by means other than simply trial and error.

Once you're familiar with some of the lanugage and parameters, you can then approach the designer to figure out whether or not he was smoking something when he built it . . . and maybe exactly what he was thinking.

Oh and for a crossover, an Ashly XR1001 is a great place to start. They're WAY better than the cost suggests, the manual is very well-written and explanatory, and it's made in the US . . . not like the cheap Behringer crap.