Don't know all the details, but it sounds like there was something wrong Before you set the volume control at 50%. Just because the gain setting was 'high' - that shouldn't damage an amplifier. (Speakers can be damaged like that). The 'loud hum' you first heard was not normal, and leads me to believe in a "preexisting condition".
Will high volume setting blow an amplifier?
I was recently adjusting my new interconnects and checking the grounding,because I was getting excessive hum through both channels. I had the volume knob turned to half way so that I could here any reduction in hum while checking the connections,I realize now this was a big mistake due to my ignorance. My local stereo shop told me that there was excessive gain into the amp causing it to blow. My new Rotel amp ,is now on its way to the repair shop. Can anyone please explain to me more about this?