I thought I would share this post from kimlajoie.com about the differences between different audio sources can be so minute that one cannot discern them with normal listening alone for my daily audiophile blog roundup. In these cases, an ABX test is necessary to differentiate between sources. ABX testing can be used to differentiate between different levels of MP3 encoding. ABX tests can also be used to test out amp sims, where the difference in quality can be heard only by particularly discerning listeners.
Worth your attention:
Audio perception and ABX testing