Hi
As unksol says swap the subs over to see if it is the sub or the amp.
If you can open up the sub and check the wiring to the speaker driver it is not unknown for the wire where it attaches to the terminals on the speaker to come off as there is a lot of vibration inside the cabinet.
Place a thumb on each side of the diaphragm and press in gently,if it feels rough or gritty then it is probably blown.