$600 for two subs??? DANG, YOU ARE GONNA HAVE A KICKIN SYSTEM THEN!!! i would say for $600, definitly get 2 12" JL audio subs. they are the top of the line IMO. a couple of W6s will hit so hard you wont know what to do about your headache! of course you can always just turn them down. i dont know if the $600 included the price of an amp, but if it does, then forget the JLs, they would be too expensive.
check: www.savinglots.com www.caraudio.com
JL is a really nice sounding sub. Keep in mind when you say the "best" you have to ask yourself, the best for what? There are subs on the market that are designed for excursion.. and there are other brands that are designed mainly for SQ.. and won't hit as hard so when you say below $600 you have to ask yourself... what do I want?
Do I want a sub that will blow my windows out, or do I want a brand that is designed for optimal SQ, but at the same time won't come anywhere near busting the windows out. Basically what I"m saying is that there is no "Best" brand out there because each company has different goals.
2- Phoenix Gold Ti 12's (sealed) w/ a PG Tantrum 1200.1. you can find the amp for 350-400 and the subs for 120-150 each. excellent sq that can get loud.