Even in the realm of real devices, real components, and real effects, differences are often so subtle that you can hear them- really hear them- only if you have a very good ear and lots of listening experience. The differences are so subtle that double-blind testing- the standard method for distinguishing real effects from imaginary ones- has been rejected by much of the audiophile community as useless, except in the most obvious cases. This rejection is justifiable- even statisticians agree that DBT misses some subtle effects.