Lets say I just bought a 2K3 Maxima and can't really spend a whole lot of money on audio stuff. I want to keep the Bose head unit and the speakers, and just get an amp and subs. What would you recommend I get, say, with a price ceiling of $500-600. I like as much bass as possible.

 

A ported 10 or 12 inch sub in a home built enclosure. Matched to a small class D monoblock amp. 2 gauge welding cable for power and ground.

 

Here's a link for you to take a look at: Link

 

Don't forget to check the used equipment market. It's amazing. There are at least as many things for sale used as there are new. And the prices are of course much better. Someone else decides they want to upgrade or need money and it's all for your benefit. MaxSE03 already gave you the link to ED. I will provide a few more lesser known quality companies links:

http://adireaudio.com/
http://ddaudio.com/
http://www.respl.com/
http://www.audiomobileinc.com/

 

Look at my setup on my homepage. I spent around $400 and installed myself and it hits hard and sounds good.