Poor sound unless faded

[Ewr101]

Simple stereo set up, nothing special- head unit feeding 4 channel amp feeding front components via crossovers and rear 6x9's. Had the Blazer in the shop for some work requiring the body be lifted off the frame. Of coarse the interior had to come out to accomplish this, including my stereo amp. Driving home after picking it up, the stereo sounded awful. And I noticed a pop when turning it off that wasn't there before as well. I rewired it correctly when I got home but it still sounded weird, better but weird and pop still there. I wanted a more powerful amp anyway and from what I could tell from researching online, that was the most likely cause so I got a new amp. I wired it up this morning before the Texans thumped the Bears (sorry, had to plug my team) and still no change. I've verified the polarity on all the speakers, well just to the crossover for the fronts, (I don't THINK they would have taken crossovers out or even need to touch them) the amp, RCA cables are plugged into the correct ports, reset the head unit and everything I've checked is correct. And here's the thing, if I fade the sound to either the front or the rear, the audio quality goes way up! With the fader set to zero, feeding all speakers equally, vocals are barely there, no bass, and it seems as though there's an echo to the music. Any help would be much appreciated! Thank you