LC-Circuits and Feedback into power supply
It sounds like the power supply on device A is susceptible to noise, and might be poorly designed.  Noise can get through power lines, especially for anything that is switching power quickly.

If, and only if, you're certain that proximity has no effect, I'd go for something like an AC power conditioner.  This device is relatively inexpensive, but there might be something like it that is medically certified.