This is a good read through from headroom on it.
Here's a great video from Tyll at innerfidelity showing how these measurements are taken, and what they can tell you. I have this set to the point where he talks specifically about the 50Hz wave. Great two part video IMO. He also shows you how adjusting headphones and seating them properly really changes the sound.
Weird how the AKG 70x series seems to have that happen. I think at some point someone had religiously recorded the time spent on his K701 and said at 1500 hours there was a huge change.
Yes - his brain broke.