I'm not even thinking ss or ultra so those are out of the question. The CS+ seems to be the best route if your going with 2 (doesn't make sense to buy a 1000 amp and use only half the power.) I am leaning toward (2)CS+. PCI could be a good move but the pci plus is right around the corner(what will this include? A bigger amp? Different driver?) And as far as freq. I have no idea. Maybe 20 would be a good tradeoff. 16 is low but not as loud. 25 is higher but louder then the 20. Those freq. are felt more then heard anyway. One last thing which model is the most recent CS+ or PCI thanks?