We need to know more about the rest of your proposed system.
The problem of goinn up higher than 300hz (within reason) is not so much the driver but the effects of colouration from the interior of the enclosure back through the woofer cone. Larger boxes because of the panel sizes and internal dimensions produce box colouations. The normal use of bracing will help control panel resonance in the bass region and the fill (fibre glass damping) on the walls of the enclosure will absorb a lot of the energy and provide some degree of damping of the woofer at box tuning frequency.
However you should consider boundary layer pressure damping on the walls of the enclosure if you propose to run above 300 hz. Even at 300 hz crossover may help.
Sometime back I experiemented with this and had excellent results using bubble wrap on the walls and styrene pellets on the interior floor of the enclosure.
The 2245H was designed for a response without the need for boundary reinforcement like some of the other JBL 18 inch woofers. The 2241 is a different applciation of driver and its response below 100 hz is supported by wall boundary effects. The 2241 has higher sensitivity in the upper band area above 100hz but does not have the extension of the 2245 in terms of normalised amplitude response.
You may therefore find the placement and effects of room gain are important to obtaining smooth mid bass response .
Consider this alignment from Drew Daniels box calculator for the 2245H which I recommend for home use.
10.15 cuft 3 (net vol), Fb 27.3, Fs 32hz. The response is-.5 db down at 72 hz and -1 db at 46 hz and -3 db at 32 hz.
I have used this design with outstanding results biamped from 250-400 hz using an Audax 210HTO mid cone driver with the wall damping methods discussed above.
Ian