New stereo sounds awful

Just had my 2008 prius with the JBL system upgraded to a aftermarket stereo. Ever since the new stereo was put in the audio is just unbearable, it's super quiet and if I turn it up past half way the speakers distort horribly. I have used this same stereo in my 2006 eclipse with a stock rockford fosgate setup and it sounded fine. I've tried tweaking all the settings in the stereo, nothing helps. Is is just something to do with the AMP that the car comes with? I had a shop install it and I know they used a harness to get everything connected properly. Only thing I can think of is they somehow just bypassed the amp and it's literally just playing right from the head unit now. It literally sounds worse now than it when it had the factory stereo in it. Once thing I did notice is the stereo has a internal amp button in the settings, and if I turn it off I get no sound, which is what makes me believe they just bypassed the amp.

 

Thanks, I went back and checked, they didn't bypass the amp. So it sounds like they didn't adjust the amp. Gonna be honest, I've had horrible experience with shops and don't trust them to do anything right. Does the amp simply have a knob or something to adjust? I do know how to access the amp but I didn't see any adjustments.

 

It actually turned out to be the amp, audio died completely, took it back and they tested it and the stereo is putting out sound, but the amp is dead. Ended up just getting a replacement OEM on eBay for 65.