Is there a DIY design/exploration of a sub 20 hz 15"-18" Tapped horn?

Multiple cheap woofers won't have to work as hard as a single expensive woofer, so, distortion may not be worse. And, low frequency sounds often have plenty of harmonics already, so the subwoofers' contribution may be negligible.
But I guess some surrounds and spiders may be noisier than others, so it'd be wise to test a sample before ordering 10.
 
Good plan! I was interested in some cheap JBL subwoofers on sale and a neighbor wanted a BP6 for his Honda, so introduced him to our modern version, the TH. Quite impressive in his car, but it highlighted why it was cheap/on sale, so passed on the entire genre figuring his would likely be the best of the bunch.
 
Did this model YESTERDAY on avsforum.com.

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Similar to these enclosures with 2 speaker cutouts.
 

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