So I upgraded my stock speakers to kickers. But they sound like crap. I still have the stock head unit and after some research I think that is the culprit. However, I have upgraded speakers to stock head units in the past and they have never sounded this bad. I am planning on replacing the head unit but I want to know if anybody has experienced this?

 

I replaced mine and they sounded like crap as well. It turns out they weren't sealed well.

I ended up going to Walmart and buying some foam sheets from the craft section and the cheapest pillow they had. I stuffed the soundbar and dash speaker buckets with the pillow stuffing, and used the foam sheets to seal between the woofer surround and the buckets.

After that, perfect, enjoyable sound.