do you know how to set the gain? did you use a DMM or an o-scope? if you need a lesson on how to set it i can help you out. but yes, when it stops getting louder, its because your clipping. it means you set the gain on the amp too high.
but, if you dont believe me, turn that remote all the way up, go full tilt on your HU and play it for 10min. i bet you'll smell something stinky.