crescendo and sd 3k's are pretty much identical, both are beasts, but the crescendo is 550 new and sd's are usually more even used.
as for the ap3k's, they can be a very good choice for the money. we clamped a pair and they were doing a little over 2900 a piece at about 13.8v. so if you can keep the voltage up, they will do rated. as far as reliability, i know someone who's had one for about 2 years i'd say and it's been rock solid.
edit: i'll also add that in a straight drop in, 2 crescendo 3k's outperformed the ap3k's by close to 2db's on a 30 second average.