I have a JBL 12" Sub that has worked great for a few years. The other day it started making a popping sound and after a few minutes it got really really muddy and boomy then quit. I turned it off and checked to see if the kids had stuck any tonka trucks in the port. After an ho ur or so I plugged it back in and it seemed to work fine for about 10 minutes and started the same thing. Would this be an amp or speaker issue? Thanks for the help.