Well I had a system that just didn't play bass I changed the receptacle s to the 20 amp lines to the amps. Wow I got a bunch of bass. I am wondering if you cannot tell much difference because you have a problem farther up the line. If you don't want to spend to much money try a hospital grade plug. You can hear the difference between the plating on the receptacle s as well I have both gold and rhodium receptacle s. Try cords from a friend or try cords that you can audition and return.
Regards