How many amp fuse(s) is in the amplifier? That can give an rough estimate of max power. You know if it is only a 20 amp fuse the most you can get out of it is less than 300 watts.

For 2 dual 4 ohm speakers you will want a 2 ch. amp running 2 ohms per channel to get the most power out of it. For 200-300 bucks look for a Kenwood x-621. Plenty of clean juice for those subs. I have the mono version running my 3 JL 10 W3v2s. Sounds good. The best option would be a JL Audio 500/1 but new it is a little out of range, retail is $550, but no other amp will as good as that for the money. JL amps are arguably the best. Also, if you have a local shop build you a box have them build a slot ported box. You get plenty tightness and impact as well as smooth low bass. The problem with that is box size. It will be much bigger than a sealed box. Regardless of which you get, get some good power to those woofers and you will be rockin'. W3s are good speakers.