Same story, I have the same problem. Now the only difference with my problem is that applying pressure downward onto the Main Volume knob fixes the volume for both speakers. The knob also has a lot of wiggle room which shouldn't be there. I'm going to open up the control module some time today and investigate the problem. Hopefully something doesn't need re-soldering. Reading through this thread, it seems like a new control module unit would solve my problem.
I had these speakers for 5 years, and its just this year I'm experiencing this problem. Best speakers I ever bought 😎