i wouldn't get the nx4 amp. there was some reviews that was saying its not that great. look at "Precision Power P900.4".
as for getting the ck's or the rk's, i'd say get the ck's because they are more sq oriented and the rk's are more spl. not only that take a look at the mounting depth and the rk's are 2 3/4" deep so that may make it difficult to install. a
for the sub i would say a skar IVX-10/12 d2ohm in a nice ported enclosure ported to around 28-30hz
as for getting the ck's or the rk's, i'd say get the ck's because they are more sq oriented and the rk's are more spl. not only that take a look at the mounting depth and the rk's are 2 3/4" deep so that may make it difficult to install. a
for the sub i would say a skar IVX-10/12 d2ohm in a nice ported enclosure ported to around 28-30hz