I have serial 20433. It has a weeee bit of breakup at the dredded 6.5k, but I could be imagining it. That was with a 6.5k test signal through a Lyr. But that was also with brand new tubes on the brand new Lyr on brand new phones, so I can't vouch for my test conditions. The headphones don't have more than 100hrs on them yet. No spring sound from mine either.