It depends on how you wiring them.
(assuming you have 2ohm coils)
If you do the voice coils in Series / Parallel then it'll be 1.33ohm. For that you'll need a 2000w amp b/c they'll be at 1.33 ohm.
If you wire them Parallel / Parallel they'll be at .33ohm. You could probably get by with a 1500w amp, but good luck in finding one that will support that draw at such a low ohm!!!!!
Seriously... I run my 2 10" l7's off more than 1300watts. You're way under-driving them if you use a panzy little 1500w amp. Those subs are just getting good and started at 600w each.
You really need to spend some time learning about audio. Go poke around at the
kicker subforum of sounddomain.