Car Subwoofer Concern

[Toniwu]

Ok, I have a nice monoblock amp pushing 2 12"s. They used to slam like no other. Then they got stolen. The insurance company provided me with some new 12's to put in their place, and ive used those for awhile, but i dont like them nearly as much. Then, miracle upon miracle, the cops arrest the people that broke into my car and i got my old subs back. They dont look like theyve been tampered with in any way. Problem? I put them in my car, and they dont hit half as hard as they used to. Ive tried all kinds of different amp settings, and tried to fine-tune it and fix the issue. I can only think of a few possible causes. Could it be possible that sitting in single digit temps for a few weeks has made the too stiff? Ive never heard of this, and i tried warming them up for a few minutes with low-volume and no change. didnt know if this could be a factor or not though. Also, would it make a difference if i mixed up the + and - from the amp to the subs? and if so, would that be the cause? Im running out of ideas, and i really hate to get rid of my beloved old subs! thanks for any help.