How many JL's are you using? If it's one or two, you probably don't need the big of an amp - although too much power is better than no enough... (it's not enough power that blows subwoofers)

The Theil/Small parameters of that speaker suggest a sealed box for the most accurate reproduction of music, but you could use a ported box without too much compromize in accuracy. You will definately get more volume out of a ported box (3 - 6 dB increase - where 10dB would be twice as loud to the human ear)

Most individuals would not notice the performance differences (especially sound quality) between a properly build sealed or ported box, so the significance is minimal.

THE MOST IMPORTANT PART OF YOUR SUBWOOFER SYSTEM IS THE BOX!!! Not what kind of box, but the design itself - ported more than sealed.
It's very important you have a box made specifically for the drivers you are using. Don't place the subs in ANY ported box. The port itself acts as a tuned resonator in direct correlation with the characteristics of the driver. Wrong length and/or diameter of the port, and your box will sound like CRAP... and I mean really bad.

1. Use a box designed by JL Audio
2. Have a custom box built by a QUALIFIED institution

Good luck with the system... those JL's should sound fantastic!

acurabros.