Big bottom? Which Kosmos too? I like the older blue kosmos better than the newer silver/white one personally. You should really start a new thread for this- it might bring you more help from other members etc.
It's a bit of a hindsight: As far as tapped horns/ rear loaded horns are concerned, the bottom slash front part you enlarged is where the efficiency is "strongly located".It's that strong (let's make it 2 points) that it will easily over power other characteristics of the cabinet, such as the low frequency extension. If you look at the part of Cubo 18 for instance and directly compare it with Cubo Sub, you'll see that though it is larger for Cubo Sub, it is only a small portion of the overall increase in size. Throat size (1 point), chamber size (1 point) and horn length (1 point) were all increased, all for extended low frequency. So ~2 points for effciency and 3 points for low frequency extension.
Beneath a certain mouth area, percieved throw and other charateristics are affected negatively. Once you have reached that border/ "" threshold"", increasing the horn mouth (1 point) will mainly add efficiency at the upper end of the usable bandwidth, slightly at the expense of low end extension (unless you'll upgrade low end extension by 1 point to counter balance). I suppose the roll off above 100 Hz is the dip associated with rear loaded horns (above the upper end of the bandwidth).
4520 looks like a monster for Sub bass between 30 and 60, but I like how the Bush bin does not have a dip until above 100 which is a perfectly respectable range for a bass cab. The dip at 75 hz on the 4520 is quite drastic. That would be a phase cancellation between the horn loaded signal and the front driver signal (right?), so it would be hard/impossible to eq that out.