blown sub?

take the sub out and put a DMM across the VC.  See if you get a reading.

if you have  a DMM (digital multi-meter) set it to the ohm setting and connect the positive terminal from the sub to the red probe on the meter and do the same with the negative terminal.  You should get a reading ranging anywhere from 2 to about 8 ohms.  if you get numbers jumping all over the place...200, 80, 2,17, 660 etc....or it says OL, then it's bad.  another way to check is to get a 9v battery and hook the positive from the batt. to the positive terminal on the sub and same with negative.  if the sub moves out then you should be good...if it moves in then you have your wires backwards, but should still be ok.   try those...  ...half of the truth can be worse than a lie. <----Roger Russell said that..