I have found that as your system gets better with better equipment, what you thought was mediocre gets better too.
The problem with the "garbage in = garbage out" argument is inaccurate . IE, What happens when you pour pure water into polluted water? The pure becomes polluted also. This is what happens with mediocre systems. how do you know that the mediocre music you hear is a product of a mediocre recording or mediocre system?
The only way to know is if you hear it in a great system. But who takes their mediocre music to a $100K system? Few, if any