I think there are enough reviews on this website and generally on the internet so I won't be long in this one: these headphones are absolutely incredible for their price point. In fact, even at $500 they would still be interesting.
You can deep lows with a lot of slam, slightly laid-back mids and smooth trebles - but no Sennheiser veil here.
The headphones are easy to drive, and you can get a smooth sound out of them with a tube amp if that's what you like.
Loving' the new Sennheiser HD 6xx open headphones from Drop. I have a small (but mighty!) sound system in my home office: Aiyima T-10 Headphone pre-amp to Aiyima T9 Pro power amp to Sony SSCS5...