It just occurred to me that the second bass can likely be brought in, such that the "baffle step" is compensated for. Thanks for bringing up the 4435:s.

Furthermore, I have on the roadmap to explore using different tunings of the top and bottom cabinets. Add to this the possibilities for directivity control, and maybe even avoidance of energizing vertical (odd-order) axial modes...

Conclusion: Dual 15":s are way cooler than a single 18"