However, as this is DIY project my comments were tailored for standard 2-3 way speakers in a simple box that are suitable for a DIY project. Helsinki has a unorthodox shape but it is a passive speaker. On the other hand both D&D 8C and Kii3 are extremely complicated speakers with multiple amplifiers. Kii3 has five power amps driven by an integral DSP in each speaker and has a remote control! Neither designs are within the realms of a DIY project.
I tend to agree that it might be worth another look. However to lower complexity, one probably would like to keep the number of channels per speaker to 3 or 4.
3-4 channels of DSP per side for each speaker makes sense because affordable 8 output devices are fairly common.
Four channel DSP plate amps are rather rare. Therefore, it would probably be desirable, if possible, to limit the project to 3 channels.
This means that only one DSP channel is available for the bass module.
If I interpret the thread so far correctly, then the bass module must not be too wide (0.25-0.30m?), but still deliver a lot of sound pressure level and still control the radiation in a certain frequency range.
In order to minimize a possible directivity error at the crossover frequency from the top speaker to the bass module, we need to think about up to which frequency the radiation by the top speakers can still be controlled.
For typical cabinet shapes directivity is controlled down to 700-1000Hz for a 2-way top, for untypical shapes it might be possible to control directivity (to some extend) down to 400-500Hz with 0.3m baffle width.
Now I'm wondering if it's possible to design a bass module that requires only one DSP channel and allows some directivity control using passive elements (slots and-or passive components).
This actually leaves only a bass module with slots or a module with offset drivers at the side or rear (open baffle has already been ruled out as far as I know) - i.e. a 2:2 or 3:1 arrangement.
Here is an example (sectional drawing) of a 3:1 arrangement with three 8'' woofers in the front one in the rear.
Or am I missing a simple solution possibility?