Bill, have you tried calling Polk customer service? They can assist you with the MP1000. I used to have that sub back in the day but it has been to long for me to remember the things to look for when troubleshooting it. BTW, call. Don't e-mail.
Also, you will not have to ship the entire sub. If the issue can not be isolated and corrected over the phone, all you have to do is unscrew the plate amp and send that in. You don't have to send the entire sub. If it was purchased under an authorized Polk dealer and it is still under warranty then it won't cost you a dime to repair besides that of shipping the plate amp. If it isn't under warranty, then sign up at Club Polk and mention that you are a member of the forum to receive a discount [I think it's 5%...not sure] on your repair costs. Either way, this should help you to get it back up and running. It's too beautiful a sub to be just collecting dust.
In search of accurate reproduction of music. Real sound is my reference and while perfection may not be attainable? If I chase it, I might just catch excellence.
The best way to enjoy digital music reproduction is to never listen to a good analogue reproduction.