I currently have the AKG K712 for the past 6 years... struggling with whether I should buy the HD800 or HD800S or buy nothing. Im almost 100% movies. Wanted that big bass impact you get from a sub or something similar. Already ordered A16.
My 2c: wait & see what you feel the A16 needs once you've heard it. Also, it's not out of the question to run a subwoofer along with open-back phones, surely? I'm not 100% sure of the routing in the A16, but it would seem to be an approach.