The important part of Bigshot's question is whether you've actually done it under controlled conditions - blind, level-matched, etc. It's also easy to tell the difference between two (otherwise identically-performing) sources if there's even a 1dB difference in levels between the two sources. It's almost impossible not to hear a difference between two identical sources if you're getting up, plugging in a different source, going back to your seat, and hitting play 30+ seconds later, especially when you expect to hear one.
In my personal and professional experience it's actually harder to convince yourself you don't hear a difference between two device, than identify a "clearly audible" difference. For many of us it's a very dispiriting result, psychologically. In our minds we want to find something, finding nothing is no fun.