I guess it's stalled waiting on me to do more stuff. But I'm waiting for the bandpass bertha plans to appear first, which is itself waiting on the new site I believe.
Of course, opinions/ideas about such a sub are always welcome, and should go here. Volunteers for prototyping/testing etc are also welcome to make themselves known. Unlike the MDP, this will be small enough that I'll be willing to build one prototype (in fact it it works well enough I might sell BPB and use one of these instead for my small rig). However, a minimum of 2 would be required for testing, together with some HD15s of course. Preferably 4. And a venue would be needed.
I need to be reasonably sure there's interest and people will help out because modelling it requires a compound model that incorporates SD15, HD15, amplifiers and controllers in order to show a whole-system response. That's several days' work.