2000 Jeep Grand Cherokee limited door speaker problem?

I have a 2000 Jeep Grand Cherokee with a infinity premium sound system. My front two door speakers stopped working the other day so I figured they were blown. I replaced them but still nothing. I have the infinity amp wired to them, which came with the premium sound system. The back two door speakers work prefectly, but when I change the balance to just the front speakers, it's very muffled and you can barely hear anything.



I think one of the channels is blown in the amp, please help.