Here's a tip which costs nothing, nada, and you will hear a difference. Just cup your hands behind your ears. Even if you like the sound I know it's a real pain keeping your hands in place but you will hear a difference. I recall that too many years ago to remember a US company tried to market a cardboard device which looked like Mickey Mouse ears and you fitted to your head when listening. It looked ridiculous and therefore didn't sell but it did work.
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.