first of all im sorry for my late reaction i was very busy lately
and i want to thank you for you troubles
second i understand some of it but im a little new to this so can you explain a little more in easier understandable language ?
Looking at the graph, you can draw several conclusions. At 30Hz the predicted response of the Monolith sub is 6dB stronger than that of the XXLS400. At 20Hz, the difference is 8dB. This is in spite of the fact that the sealed sub has more watts than the vented. It's clear that the Monolith sub is more efficient than the XXLS400, as is generally the case with all subs with similar drivers. And a sub that can do more with less power maintains more in reserve for the peaks. If the XXLS truly is flat to 23Hz or so, most of its range must be severely attenuated to meet its max output at that frequency. In other words, if its max output at 23Hz is xmax limited to 99dB, then BK Elec would have to limit the rest of the range to 99dB to keep its response flat. The Monolith on the other hand would be flat to 25Hz at 108dB, so it's capable of playing louder.
In practice at normal listening levels what this means is that when the sub is asked to reproduce a kick drum that's 3 times as loud as the bass line, the Monolith is more likely to have enough power in reserve to handle it, whereas the XXLS might compress the attacks. And of course, movie explosions can be felt, not just heard.
The only reason the XXLS might be preferable is if you want a sub less likely to bottom out on frequencies lower than 16Hz, or if you absolutely can't spare the space needed for the Monolith's added cabinet size. I say all that to say the Monolith sub would most likely be a better choice.