To the guys that read this, i need a little help or tips on why my subwoofer has distortion. My set up is

Alpine CDA-9857
MTX Thunder6500D Mono-Block (2 ohm Load 500w rms)
JL Audio 12" W6v2

OKAY, heres the deal! The Alpine has a Subwoofer OFF feature. When i turn the subwoofer off using the Alpine head-unit, i noticed that the subwoofer still jumps a little bit, and at high volume it jumps more when its suppose to be off. Is this normal? Im thinking its DISTORTION, but im not 100% sure. The only thing i think of that would cause this is a bad ground for the head-unit or the amp. I checked both of them and there both attached to pure metal, no paint in between at all. IF ANYONE KNOWS WHY THIS IS HAPPENING... PLEASE HELP!

 

my guess is the sub is just vibrating from the sound produced by the
other speakers - an easy way to test this is to physically disconnect
the sub, if it still moves that's it. otherwise I'd guess the circuit isn't
completely isolated in the headunit itself, and a small amount of
current is bleeding over to the sub cicuit even though it's off.
either way it doesn't sound like a big problem.