which sub?

[05 obs 0]

alright so i have a sundown sae-1000d. i am currently running 2 type r 12's and they just dont do it for me. which singe sub should i run? i was looking into a single 15 or 18. i know 1000 watts isnt too much but its what ive got to work with so any help is appreciated. budget isnt too big of a concern since i will be selling the type r's.