I have to agree. I don't use my X2's as there's something off which the Senns do just right. It's in the mid/treble region I don't get a long with, so I just don't use them.
Really I just want the 600/650 with more bass extension. That would be the perfect headphone for me currently without getting into the ridiculous price realm.
It's the cohesion between the mids and treble it's flawless in how it transitions, they just sound right, like what music is meant to sound like. Most headphones these days recess the upper mids where as the HD650 has always been more forward in this area; gives female voices that body and range. I've been waiting for a headphone with HD650 timbre, tonal balance and improved bass for years. The Focal Clear gets close but the timbre is completely different, more metallic harsh. I've accepted it'll never happen despite many headphones like the ZMF range trying to mimic the HD650's.