This sure is a whole lot of work, expense and fiddling around for a general purpose cab. Build two cabs, buy two 3012LF drivers and a $135 6" driver. Expensive. Build a passive crossover for the one cab, and a LP filter for the 2nd. Expensive. Use a second amp channel as needed. Expensive. All this effort and expense results in two cabs that produce 100 SPL for the woofers and 102 SPL for the mid-range. Those wanting two cabs are better served with a modest 6" driver SPL matched to the 3012LF driver. This is the Alpha 6. Build the baffles in mirror image layout, so the 6's are adjacent when stacked. Unlike the 6ND410, the Alpha 6 can crossover at 800 Hz and not run out of linear response. PXB2:800 works just fine. The passive crossovers in each cab remain undisturbed with one or two cabs in use. No dual channels, no LP filters, just plug and play. Less expense. Those wanting a single cab that makes 100 SPL are better served with the 3015LF and a matching mid-range. I am not aware of a 6" that is an SPL match to 3015LF, unless horn loaded. That is already done, and is a proven and measured solution. Non-horn loaded, two Delta Pro 8B in parallel is a power and SPL match to 3015LF, and uses a standard crossover. Yes, the 15" box is larger. For this project, the second box could be retrofitted with a 6" driver unless there is a brace in the way. The biggest PIA is cutting the baffle. The sub chamber can be easily fitted through the 12" driver opening.