I've had my k702s for about two years. Right out of the box they sounded thin and strident, with lots of detail and commendable soundstage, but they didn't sound right. They had little bass, a tizzy upper end and a sound that was definitely fatiguing after even ten minutes of listening. After 300 hours of burn in, it's like a different headphone altogether. The harshness disappeared, the tonal colors became rich and deep, the soundstage now populated by flesh and blood musicians playing their instruments.... Not only was it not subtle, it was night and day to my ears. Instead of being fatiguing, I can now wear them all night at even higher listening levels.
Just about everything in high end audio sounds better after burn in: tubes, transistors, cartridges, speakers, headphones, and even cables!
Measurements are useless in proving or disproving this phenomena. Like the saying goes, "if it sounds good and measures good, it's good. If it sounds good but measures bad, you measured the wrong thing."
The K702s DO need burn-in. It wasn't until I burn-in my over 100 hr. mark, that they came to life. Very good headphones.