This is a sketch of the Mk2 version of this cabinet. For me this was a novel approach and I wasn't sure if it would work as intended. I have seen the opposed drive units approach used in more modern designs (notably the LAB sub) but I don't know if it was used prior to 1978 when I devised this cabinet.

The graph below calculated by horn response shows a remarkably smooth response. I was expecting it to be far worse. With two 50W drive units the remarkable 110dB sensitivity was needed. The cross over points were 200Hz and 1200Hz