Help with subs, not hitting same time. <3

Ok, i was going to sell these subs but iv noticed its having an issue that i thought went away

When the bass hits they dont both go out even. They both go out at the same time but one goes out more and ***** the other one right in. Its ok on medium volume but high volume it makes the weaker one clap.

I tested with a 9v battery and they are not out of phase

the box is 100% sealed and even made a silicone gasket underneath the subs for a complete seal