I can't take the credit for the design of these. Another forum
tosses around the TABAQ (TAng BAnd Quarterwave) design (mass loaded transmission line) as one that works well with a wide range of 3" and 4" Tang Band extended range drivers. In my case I used them with the new W4-1320 4" bamboo drivers
They are still breaking in but the sound is remarkably flat with decent bass extension (for a 4" driver). It is a very pleasant and non-fatiguing sound. Vocals are incredibly clear. It does lack some mid-bass punch for hard rocking out - what do you expect from a single 4" driver. I have noticed that they are picky about stuffing - some tuning is required.
I'll post more as they break in. . .