Sounds like an out of phase speaker somewhere.

That's a good point. When I first got my Accord, I had wired one of the rear deck speakers out of phase, and was surprised how awful the bass sounded. It wasn't subtle, it was very obvious.

I discovered the polarity problem almost immediately because I shifted the balance left and right, and noticed that the bass sounded fine when the balance was to one side or the other, but awful when it was centered. That was the give-away for me, and it took me only a minute to find and correct the backwards wires.