Hello Wayne!
Tried an experimental prototype sub using a pair of Lab 12 in push pull configuration. Non isobaric and reminiscent of M&K design.
About 83 litres cube with typical amount polyfill damping.

Powered it using BK elect.England subwoofer amp rated at 500 rms.
The SPL and extension is astonishingly good, Also a 12db boost at 45 Hz using Bassbox Pro simulation gives a perfectly flat in room response.

The cube measuring about 21 inch is very small and ideal for home use.
Considering builing a pair using 36mm MDF, fully braced and damped.
Await your advice and help in the project.

Roland.