Stereo troubles
 

I have a 2001 Monte Carlo ss and I recently had a local stereo shop install a Sony cdx420-ip head unit in it. I got it for free so why not. I bought the PAC rp5-gm11 wiring interface for the install and ever since they installed it, it has never sounded as good as stock I put in new door speakers and rear deck speakers with no luck. I also bought an amp bypass and that just made it way quieter ( as in I had to turn it all the way up to even hear it ) did they install the wiring interface wrong? Or is it possible that this head unit is worse than the stock I have read great things about them online so I don't think so, by the way the head unit was brand new when I got it. Any help is very much appreciated