I am having some problems with the RT20 speakers and a center channel I purchased new in 1996. I now have 3 drivers that have a humming sound. There is no visible defect in the rubber surround or the cone. This makes 4 speakers that have had this problem. One was repaired under warranty. The retailer I bought them from is no longer in business so their service department is not an option. Two of the problem speakers are MW8003 and one is a MW4850. What are my options? Replace the entire speaker system? Replace the drivers? Repair the drivers? I like the speakers and would prefer to keep them if it is practical. Is this a common occurance with these speakers? I hope some of you will take time to guide me. Thanks in advance.