Thanks guys, appreciate the comments.
Trey, you are probably correct that I probably overdrove it. I have a Denon AVR 1803 receiver that I connected the sub to via the LFE channel. I have always left the knob on the sub at about the 10 o'clock position, but I do crank the master volume on the receiver to -13 which is pretty loud. The sub only makes the popping sound on a heavy bass passage like when a cannon fires and it really extends the speaker cone. The pop noise that the speaker makes is like a gunshot noise, like something is arcing inside. That's why I wondered if the voice coil may have rubbed the insullation off and is shorting out on extension of the speaker cone. I did take the speaker driver out and looked it over for any obvious damage, but did not see any.
I'm not sure who I spoke with, but I may try calling back tomorrow and ask for Steve Phillips. I did talk to someone in the parts department and they said the K-26-K driver is still available and they had several left. They quoted me a price of $149 plus shipping. They did say the amp was no longer available and I didn't think to ask about the passive.
If I knew for sure this would fix the sub, I would would spend the money and buy the driver. I know there are newer subs out there that may be better suited for HT, but I bought the Chorus II's and the sub together several years ago after I payed off my student loans and they are pretty sentimental to me. My wife and I have listened to hundreds of hours of music & HT played through them and as corny as it sounds, they are like part of the family standing proudly in their places and we have thoroughly enjoyed them. If it's time to move on to a new sub, I will, but I would like to give a shot to fixing this one first.
Again, thanks for the comments.