Need help with a Newcomb 12 inch Jensen C12N speaker

Hi, I just bought a pair of Newcomb 12 inch C12N by Jensen guitar speakers. They were said to be excellent but one had a tiny rip in the surround, looked like punched out during removal by the mounings screws. I tried repair with a tiny piece of tissue and a tiny bit of nail polish. The speaker sounds horrible, makes rumbling sounds. The resistance between the terminals is 6.4 Ohms (8 ohm speaker). I am assuming the speaker needs a new cone kit, but is there anything else that could be wrong? I'd hate to spend the bucks on a kit and then have it do the same thing! The other speaker is great and my Fender Twin now sounds much better with one Jensen and one Utah. Thanks for your help, Roy