Hope it's not this. I've got a couple of days left before I have to decide if they are going back. I'm trying to decide between the 551's and the 545's. After using them for a week, they are very different sounding headphones. The 551 wins in overall clarity and clean soundstage. I'm still undecided but the 545 may have a "tad" nicer midrange..it's a bit more forward. The problem I have with the 545 is it just sounds congested after the 551.
The 545's would be my walk around outside headphone but I'm more and more happy with my Amperiors for that. The Amperiors after about 30 hours of playback are starting to sound much smoother.
Regarding build quality I would say the hinges on my 545 don't feel as well put together as the 551's. They creak a bit where the 551 are more smooth and silent.