Hearing, just like the rest of our senses can be refined and educated over a period of time. As an example, top chefs and wine sommeliers have educated their palette to discern subtle taste differences that most would miss. Perfumers have developed their sense of smell to the point that they can deconstruct a perfume formulae. Or how a conductor hears the orchestra differently than the audience.
How are you hearing no difference?
In my experience, I've never heard two pre-amps that sound exactly the same, nor two DACs that sound the same, nor two amps...etc. Yet, occasionally someone will claim that they heard no difference between Product A and Product B in their system. I find it difficult to believe.