While I have not heard the HD661 yet, I will say that the HD660 surprised me a lot while listening to it during KenJam2. They matched incredibly well with the FiiO E6's red EQ setting, it added a welcome low end rumble to its default signature of having the positive Grado-ish attributes of forward excitement but without the harshness. They're more detailed and dynamic than they have a right to be at Php 2k price point, they're an absolute steal.
They may have blatantly copied the Beyer design aesthetics-wise but I dig it!