Before jumping the gun, try this:
Check the phasing of the speakers. If your speakers are out of phase you will get distortion because the speakers will be basically working against one another instead of working with each other.
How to check if your speakers are in phase:
Make sure that the wire they used for the positive is the same for each speaker. So what i mean if they so used the red wire for positive on one side but for some reason sitched the red to be negative on the other side then your speakers will be out of phase.
I know it sounds stupid but i was a Car Audio installer for almost 4 years and this is a very easy mistake to make and is probably on of the most common errors!!! Hope that helps!!!