http://mobile.jlaudio.com/pdfs/15_18_W3_MAN.pdf

Those old 15W3's required 400W RMS per sub. I am not sure if you have 2 or 4 ohm versions, but if you can get the total load down to one ohm, and you can't drop the big dollars on the JL Slash 1000/1 as aforementioned, you can get away with the TMA 1000.1 amp (still made by JL Audio). That amp is a beast for the money and we actually sell tons of them to drive these multiple loads. All this for about 1/2 the price!