From personal experience with my AKG 701's burn in is definitely necessary. I've had my 701's for about 2 1/2 weeks and put about 200 hours through them playing various music from electronica, jazz, hip hop, pop and notice a big difference in the bass. The bass started off pretty poor almost non existent. The 701's are not known for their strong bass, but I was not expecting so little, but yes with burn in, they are starting to show their true color. In regards to k701's yes burn in is valid. I won't comment on burn in for other audio equipment such as sources, or amps, cables ect. as I've not had the chance to do any trials of my own. But for now, my rule of thumb is to give all new equipment a few weeks to "settle" before making judgement.