Don Pearce wrote:

The driver is a 15 inch Adire Tempest mounted in an 80 cubic foot
concrete cupboard, which extends under a staircase on the other side
of the wall. The mounting board, which replaces the cupboard door, is
18mm MDF, braced extensively with scaffolding board, screwed and glued
edgewise onto it. It is very solid.

The whole thing tunes pretty nicely with the room to make an almost
flat response down to something very low indeed.

Flat? How does that work then?
For a closed box you want about 4 cubic feet for a Tempest.
With a very large box you want some first-order low-boost to equalise
the response.