Make sure to flick the E10K gain switch to high, they will sound lifeless otherwise.
Most importantly, give your brain plenty of time to adjust to the new sound signature. When I first listened to my HD650s I absolutely hated them, it sounded like I had cotton wool in my ears as all the high frequencies disappeared. What I didn't take into account was that my main headphone at the time was the AKG K702, which has a very different signature that focuses a lot more on the upper mids and high frequencies. After an hour or two, it may have been less because I sat there mesmerised for six hours altogether, everything clicked into place and I knew I had something special on my head. The detail was still there, it just wasn't thrust in my face, the HD650s are much more subtle (and natural) than the AKGs and if anything are even more resolving.