Spendor S5e mid-bass drive unit failure?

I'm the original owner (since 2005) of a pair of Spendor S5e speakers and have cared well for (not over-driven) them. After about two years of use a noise developed in the mid-bass driver unit of one speaker, which was replaced by RS&D under the 5-year warranty. After another 3 months the same problem developed in the other speaker and RS&D provided another driver unit replacement. Now, 4 years after the first replacement, the same problem has developed again, in the first speaker.

I'm interested in hearing whether anyone has encountered this problem. Any experience, solutions, or suggestions would be appreciated.

I apologize to the audio community and to Spendor. I have since learned that the noise was caused by one or more preamp tubes, which I replaced with the original EH 6922's. Tube rolling can be a frustrating and non-fruitful experince.