Ive had and ran both a KX2500.1 and a ZX 1500.1 on the same sub/box combo at 1 ohm and really cant tell a whole lot of difference honestly..........maybe a db.If you decide to go the 1500.1 I have one that benched at 1650@2ohms.
I really believe the 1500 picks up more then the 2500 under 2 ohms thus they arent far off from each other........but that being said the 2500.1 is a lil easier on the electrical........so chose wisely.