If you scan through data bass, you will find several subs which produce good clean sound up to 250-300Hz. You can use dual subs in stereo and locate them close to the speakers, so the location of the subs does not contradict the mains.
I would think this higher roll-off point would emulate the sound of a full-range tower, if you are feeling the bass in the 100-200Hz range is left weak when using bookshelf speakers with a sub crossed at 80Hz.