That SQ 8" is a subwoofer, and while there is no suggested frequency response range my guess is it will quickly roll off by 100 Hz even without a low pass filter.
In response to the CDT suggestion, CDT makes several midbass woofers suitable for IB use in a door, and they all play beyond 1 kHz. I always tell people to cross them higher than 250 Hz, and with HD-M6 it's fine up to 1 kHz with a small midrange. You get much better dynamics by letting these woofers play more frequencies than by putting more energy into a much smaller driver.