Re: Sensitivity.
96dB/1w/1m on the B200 is misleading. Its more like 82dB at 50hz in a sealed 100ltr enclosure. I bet you sit at more than 2 meter so that would be more like 76dB. You could well have 90dB in a more significant portion of the passband but that doesn't matter, all you've got is headroom/less distortion in those area's should you wish a flat FR.
If you want a flat response then you'll need to add a BSC network. The least efficient part of frequency response is the base denominator.
So for examples sake lets say with room gain your pushing 80dB/1w/2m @ 50hz. 12 watts would equate to a max overall SPL of 90.5dB and I'm betting the amp is over 1% THD at that output. You may have all that headroom in the mids etc. but since the amps don't have anything to give the bass and you want a flat FR, then thats going to be your lot.
You really need to be looking at an addition 2x 8" bass drivers to maintain 90dB+ in the bass range.
This is reason why folks horn load fullrangers