This calls for a test
Can el Reg organise a blind test? Given a set of listeners who claim to be able to tell the difference with their named tracks. Place them in a room with your reference audio kit of choice and play FLAC and MP3 versions of the tracks and ask the listeners to rate which was which for multiple tracks. Given all other things are equal it will only be the audio format that changes so over enough tracks we can see if there is a statistically significant result.