appropriate filtering must be used.
-high enough order
-correct Fc
-phase controls?
-Enough knowledge to use /design the filter system taking into account transducers being used.

The 15" woofer
-must be a low distortion type
-good alignment of flat or overdamped response 'non boomy '
as im sure that ribbons demand the best quality.

Woofer distortions extending into the midrange will destroy the entire sound.

see www.linkwitzlab.com -thor subwoofer.

Cheers!