Which frequency should I tune my sub box to?

i would try 35 but your amp has a subsonic all the way down at 15hz so alot of the frequencies between 35 and 15 are going to be distorted... never liked those alpine amps for that reason.not a fan of those subs either. they are more for sealed enclosures. but hey give it a try and see how it sounds at 35. just make the port at least double recommended size. i know itll be long but itll be good.