Amplifier fealure

Hello,

Today I hooked up for the first time a rel stadium v3 to an adcom amplifier model 5500. After a moment of uncertainty I finally hooked up the Neutrik cable just as I should have. Red to R+, Yellow to L+ and Black to L-. By my disbelief I soon realized that something was not right. While the right channel sounded good enough, the left speaker had no music. Actually it did have a barely audible sound coming from it.

I did troubleshoot everything, speakers, cables, channels, fuses. As result now I know that at the moment of the system power on the left channel of that amplifier has been damaged.

Can anyone experienced tell me what can I do to prevent the same happening to the next amplifier that eventually I will install? How do I know if the sub is the cause of it?

Any input will be greatly appreciated.
Thank you