My box simulation

Measurements:

Subwoofer final design with Hypex FA 501, no filters

It seems like this subwoofer has no limits in the low end bass.

Let’s check the final result:

Speaker nearfield + Bass Reflex nearfield with low pass filter LR4 65Hz set on Hypex

It shows -3dB from 13Hz (32Hz phase shift)

RED = Bass reflex nearfield

GREY = speaker nearfield

GREEN = my speakers SBS (nearfield with 22W scan speak)

Only speaker nearfield 5mm away from the cone

-3dB 22Hz!

Making: