You probably don't want to do that. The mids of HD650 doesn't need any boosting at all, and you'll just destroy its renowned balanced sonic signature and make it too bright. If you must do it, then just create a single parametric band with a gentle slope and maybe +2~3 dB and sweep it throughout the mids and find the spot that sounds the most appealing to you. Make sure you listen to a wide range of different types of recordings/styles of music while you do this, so you're not just EQ'ing for only one particular song and then all other songs sound worse with the EQ.
I agree. I bought the 650's specifically for the mids. No enhancement required.