What is the best car audio subwoofer under $500 a speaker. (internet prices of corse)?

I just bought a 2003 bmw 325i and im looking to throw some subs in it. I would like to know what would be good to put in it. I am looking to buy the subs on the internet at a reduced cost so im not talking about retail. I am looking for 2 comp subs to throw in the trunk. I need a great deal of b. I was thinking about the JL audio W7 but i was told there is better. Please inform me on your choice of speaker and where you would get it from. Thank you very much!!!! (i am somewhat new to this)