To tell you the truth, it's all about the setup in your vehicle. There are a lot of variable to consider when looking into buying any audio products. Some of these variables can be:
1. Money
2. The Amp Your Using
3. Will You Be Using A Sealed or Ported Enclosure?
4. Ohms (Single, Dual, Quad)
5. Type Of Music You Listen To
By what you stated above, i'll take it that you most likely work at a Best Buy, Circuit City, or something along those types of jobs that you could get discounts on Car Audio Products.
Personally, in my line of experience, I would stay away from MTX. Im not saying that their bad, but they are relatively expensive, and for the money, you could buy something better.
Now we are left with JL Audio vs Kicker. Both brands are very good, but there are some things that differ between the two. Instead of explaining everything, i'll just get right to the point.
You already have your JL Audio subwoofer. If you were to go buy a Kicker subwoofer, then you would have a pointless JL Audio subwoofer laying around, and would have been a waste of your money, unless you already know someone that will buy it off you.
However, if you are looking to have a loud system, then I would suggest going with Kicker. If you are looking for more sound quality, then I would stick with you JL Audio subwoofer.
