It's class AB, not class A. The problem is in the general area of the power transformer. I would bet good money that the manufacturer skimped on the transformer, and it's dying as a result.
There's also another issue with the shakers that you're using. Because they have quite a bit of mass, they also have inertia. Amplifiers normally use electric power to dampen inertial forces in speakers. Since that speaker has significantly more mass than a conventional speaker, the amp has to put out a lot more power to dampen it. You're also running the shakers in series. That doubles the effect of their inertia. Combine that with the fact that the shaker resonates at 40 Hz, and you'll easily destroy a low power amp.