BASSBOSS subs include processing that allows any combination of cabinets to be combined (on the same plane) and their output to sum coherently. This provides an increase of SPL in their common passband. A cabinet that doesn’t contribute energy below a certain frequency will not add SPL below that frequency. Combining boxes that have different output passbands, (or whose settings have been configured to operate in different passbands,) will result in increased SPL only in the range which is common to both sources. ie: The overlap range.
To give an example, combining a ZV28 with an SSP118 will result in an increase of SPL capability only in the operating range of the SSP118. Filtering the upper frequencies out of the ZV28 would simply remove the capacity of the ZV28 to add SPL in the upper range. For frequency response purposes, this is effectively the same result as eliminating the SSP118. In other words, you can add the additional low-end without creating an output peak in the upper range by adding the ZV28 and eliminating, or re-purposing, the SSP118.
That said, retaining the SSP118 and adding the ZV28 does provide the extended LF response AND additional SPL capacity, and this distributes the thermal and physical stress over more drivers. Thus, combining the boxes and using EQ to reduce the non-linearity in the response would yield a system with flat response AND greater headroom. Greater headroom generally translates to longer service life and reduced distortion. (Not to mention the ability to reach higher dynamic peaks.)
The MK3 cabinets with the ControlBASS software provide EQ filters that can be used to smooth out the response. The correct procedure would be to combine the boxes, measure the result, and apply the same EQ filters to both, thus ensuring they remain in phase, with each providing 50% of the resulting output.
Reducing the overlap reduces the SPL increase in the overlap range, so flat response is achieved when each element is contributing 50% of the energy at the effective crossover frequency. What’s not often considered is that when a subwoofer (or 3) are providing more SPL than a top, the effective crossover frequency, the point at which each passband is contributing 50% of the energy, moves up in frequency. Thus having both subs and tops with their HPF and LPF filters set to the same frequency, and their output SPL matched, would result in flat response.
This is why it’s recommended to try different combinations of filters. Some people run the subs much higher than the tops, some not so much, and everyone has an opinion as to what “boomy” or “thin” sounds like. We can’t predict preferences, and we can’t guarantee everyone has a measurement system and the skills to use it properly, but we can provide ways to achieve the results that anyone wants, without allowing changes that would cause catastrophic gaps in response, and the potential for damage to speakers, caused by phase incoherence.