The graphics card definetly can destroy the monitor if the refresh rate is not supported. I had a cheapy 15" that I ran at 85hz when it's spec was 60hz. It looked great... while it lasted (few months). One day it just died. I don't think you could have killed 2 monitors so quickly. What do you mean by "ruin" your monitor? Was it actually not working, or did the screen look more abnormal. YOu need to supply more info.
Quality is better than name brand, even regarding beloved AMD.