should I be concerned? I have a pair of Classe CA301's that I bought at seperate times. I'm not running them in bridged mode. One seems to be quite a bit hotter than the other. Hot enough that you can't leave your hand on it for more than a few seconds. The other one is just very warm although not enough that you feel you have to pull your hand away.
I would get it serviced. If an amp runs hotter than normal it can cause the transistor and other elements to fail. I agree that it's probably a bias fault caused by an open resistor, or a resistor that's drifted in value. My guess it's the emitter resistor.
You may have a wire shorted somewhere or a speaker that has a lower impedence than the other. Check the wiring first then the speakers. Switching the amps from one to the other is a good idea. One amp could be biased more into class A than the other and that would cause it to run hotter.