Ok, for good affordable sound quality go with CDT 6.5" components....CL-61a should be the part number, 150 shipped from
http://www.thezeb.com. Umms if you want straight imaging you can simply remove your rear speakers, and play with the 6.5s, so you'd be set driver wise. Is 500 your limit for the whole system or just the sub setup? Cuz 500 is kind of iffy for a head unit, speakers, sub, box, amp, and amp kit...but we'll see =) Do you want to be able to play mp3s from your head unit, or does it matter? I can suggest a few subs, Adire Audio's Shiva puts out some amazing sound if you build the right size enclosure for it, they're very inexpensive, and they play well of relatively low power. I'd say build the box yourself if you're confident with woodworking and carpetting it, it'll save you some money, and leave you with enough wood for the speaker adaptors, with some left over. A quality amp kit (4 gauge is what I've always used) will run you $30 here
http://www.knukonceptz.com/pro...OLAK4
Let me know what you mean by no more than $500, and I can help you more.