I think if you're coming from Pro900. You'll be happy either way. Signature DJ or TH600 will be a noticeable step up. I think either way you'll be raving about how good it is how you're going to sell your Pro900 immediately. So in a way you really can't choose wrong here. Also if the TH600 are anything like the old Denon line, then the sound leakage actually comes from opening between the cups and pads. You can replace the pads with anything you want, but the sound leakage will continue because of the gaps left on purpose to increase soundstage/bass. In other words the TH600 will let everyone around you hear your music no matter what pads you replace. So if you need that privacy there is no question for you, get the Sig DJ or wait for the Audez'e closed back.
I can confirm. I sold my PRO900 and live happily with my TH600.