I'm in the Death Business
For $500 bucks you could get a pretty sweet sub. Are you partial to 12's or is that just cause they're the most popular size? I went from 2 12W6v2 to 1 10W7 and I won't look back. Agreed with above that you should just focus more money towards some decent component door speakers than getting 6x9's. You'll get better sound quality out of round speakers anyways, plus you're going to have to do some custom fabricating to get those 6x9s in somewhere. Actually, firt off...what car are we talking about here cause that would make a large difference. If it's an MK3 then go with better door speakers cause the rear speakers pretty much just shouldn't even be there...but they are and they're freakin tiny. A shallow 4" is pretty much all you're gonna get back there comfortably.