I'm standing on the precipice of turning my in rotation headphone collection over and now seems like a good time to write a HD 518 review.  My HD 700's and 598's are in the mail and instead of listening to those I'm back on my back up pair the 518s.  These were the first open backed headphones I ever heard.  They are also the first headphones I ever did an ounce of research into and started my love of the Sennheiser sound.  I've owned them for several years and since then I've also listened to most of the rest of the HD line.   The amazing thing...