Well;.....found the rest of the needed info!
Don't know who told you 44 liters for the sealed box size; but that's what the factory recomends
Here's where my knowledge of subwoofers breaks down.....maybe car subs operate under different rules due to car interior sizes vs average home listening room areas
If you plot the driver, you end up with a .707 Qtc for a nice average sound in a box of around 20 liters; a whole lot smaller than what you have now.
I could see why you were bottoming though: At 240 W you're dangerously close to being at the limit of what the driver excursion/power inputs ratio can handle at 44 liters(no problem at anything less than about 40)
Anyway, the numbers I come up with suggest a sealed box size of closer to 20 to 40 liters as being safe in the home.
The company also lists the driver as being acceptable in ported enclosures as well(49.1 liters with a four in. port 16.8 in. long)
Good luck with your troubleshooting, and be sure to let us know how things work out
