I added iso-woofers to my Magnepan 3.3s. These consist of the two pairs of woofer panels from my Tympani 3's.I call them iso-woofers because the woofer panels are wired in series & mounted isobaricly. For a crossover to these isowoofers that run parallel to my modified 3.3s. I used a 1mH & a 10mH inductor in conjunction with a 120uf shunt cap for 18dB/octave at 200hz.
CAUTION: the bass impedence is 2.67W

A 6dB/octave (1st-order crossover) gave too much IM and sounded muddy...

Ever since these were added I have not been using my subwoofers. So, this weekend I removed them. The iso-woofer works so well that I feel I haven't a need for the subs anymore. There are only a few cases where the subs might add a little more bass, but not so many that I feel It's worth keeping them around.

The iso-woofers with their 12dB/octave (2nd-order) crossovers at 200Hz were doing a real good job, but a matching 18dB/octave (3rd-order) to my 3.3 woofers made the system disappear.