Sound Distortion

I recently installed a set of Rockford Fostage speakers to the front and back doors in my '08 outback. I also installed a new receiver. The sound comes out of every speaker and it sounds fine, but when I try to kick it up to a higher volume it just sounds flat out terrible with heavy distortion, especially in tracks with a lot of bass.  I'm wondering if this could possibly be the solder work on the wires for the receiver? I already resoldered the ones that even seemed loose, so I'm not sure it's that. Could it be the wire connection is loose from speakers? Or is it possible that my settings on my receiver are all jacked up. Before I tear the whole thing apart again, I wanted to get some opinions as to what is going on.