I currently have 3 JL 10w0's in a sealed box running of a infinity amp that puts out 600rms at 2 ohm. I want a louder system but i dont' have enough money to buy a new amp and the JL's just arent cuttin it. I was thinking about getting a mach 5 mj 18 and putting it in a ported box. Does anyone know this set up will work good with my current amp and be louder than the JL's?

Why not try venting the JLs first? You already have them and if you have enough room for an 18 vented, you'll have enough room for 3 10s vented.

Look at it this way; if you don't like the JLs vented, you're only out $20 and a couple hours to build an enclosure versus the money for a new amp or driver/enclosure combo.