MP418SP and 4645B

I acquired the JBL MP418SP some time ago with blown Crown amp. Repaired the amp, both channels functional. It's in the family room. Its fun to use it and on some tracks can prove its merit of being a prosub. I have read it operates between 38-150hz utilizing the lo pass filter in the amp. Is the driver not capable of dipping below 38hz by cabinet design or does the integrated amp have a subharmonic filter in the amps circuitry? I recently purchased a DBX 234 crossover to try to tame it. the DBX is currently set an indent or so below 70hz.

It does improve the sound of it being able to crossover at a lower set point. also can set amp gain at 2:00 and utilize gain control on crossover. Just acquired a 4645B and want to place this in the same room with the 418SP. Could use the second channel of the Crown amp in the 418SP but am assuming it will still play in the 38hz to whatever crossover point that is set on the crossover. if this doesn't satisfy then am thinking of dedicating an amp to the 4645B using the crossover so it can play below 38hz.

Different subwoofer models may not play well together. Whenever if you split the subs so they each handle different frequency bands they may play well together

I own two MP418SP, and the amp is basically a Crown CE2000 with a novel crossover that makes it a very versatile unit, if not heavy as you well know. The box is rather small, and the tuning is set too high to get down as low as you'd like. Most Pro Live portable subs are tuned at 40Hz, or higher, in order to maximize the benefits of port output to match the program material. Kick drum is typically 50-80Hz depending on drum size and tuning, and the lowest note on a 4-string mass guitar is 41Hz. The only instruments that can produce frequencies below 40Hz are 5 or 6 string electric bass (31Hz), a big keyboard synthesizer, or organ pedal tones. Better to have more punch and less cone excursion from 40Hz up than lower LF extension in live sound applications. I don't think it has a high-pass (subsonic) filter, but I may be wrong about that. A little online research, or measure amp output for LF output roll-off using a frequency generator or test tone CD and a volt meter. You probably could bypass it, or modify it to roll-off lower. If so, you could add fiberglass to the cabinet to increase virtual box volume, and lower the port tuning by doing so. Be careful to secure the fiberglass to provide 3-4 inches breathing room behind the woofer pole vents and the port opening inside the cabinet.