GR Research Dipole Subwoofer

One more thing... let the elements play in the SAME direction.
This will give you more 2nd harmonic distortion. But it will mask the higher harmonics (3d, 4th, 5th etc)
Sounds better this way.
Yes slightly higher THD, but remember your own ear is producing significant amount of 2nd harmonics. Up to 10% in the low frequency region. And the added 2nd harmoic is the least disturbing harmonic. Actually you prefer it after listening test...
You don't need 8 using W frames.

Tubenstein, a subwoofer is usually crossed around 80Hz or so, in order that placement can't be heard

Time now for you to reconsider repeating that old "fact" - that is based on lab tests.

On music with clean speakers and sharp XO, 130 Hz is just fine. That's a substantial benefit because then your mids don't have to struggle to get down to 80 Hz which requires much bigger drivers and cabs or OB panels.

That "80" figure is repeatedly disputed by folks who have tried higher. Try it yourself.