What an odd change of pace. The RSD12d isn't a bad low level, low frequency woofer.
1200w rms? //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif Why so much? I understand that you're giving the RSDs about 600w each, but they have rather low sensitivity and not much excursion to make good use of anything you give them. If you're going to step to something else, don't get too fixated with wattage. 500w on an efficient sub will outdo both those RSDs. You might look at a single 12" ported, something decently efficient that will get you down to 30Hz well like the RSD but better.
Did you want the same thick and heavy sound of the RSD? Or were you looking for a different sound, maybe something a little bit lighter and crisper with more detail?