JL does NOT market their enclosures without subs, as they are intended to be properly matched speakers and enclosures. The "universal" enclosures on the market are just that, and there is no control as to what speaker is put in them. Most people don't know the difference, as a 12" sub is a 12" sub, whereas quite the contrary, there is a world of difference.
We do have the JL unit available at a very competitive price, and that will be the best sounding, highest quality combination you are going to get with your given space that will be sacrificed. JL does their homework when it comes to the proper enclosure volume and speaker choice, so you won't go wrong for sure.
With others, you will most likely be compromising with either the enclosure size or speaker itself.
Let me know if you have any other questions on this or any other audio topics. If you are interested in the enclosure you can email me directly.
Todd Matsubara
TM Engineering