I have the 250Ohm T70 in hand now and find it superior to the DT770. I jsut find it's not worth nearly double the price. I find they are good around $300, but not the $600 MSRP. The T70 I have has more bass than the 600Ohm DT770 I used to have. They are more detailed and resolving. They also have smoother treble than the DT770. More mids to boot as well, but still neutral.
I find all the DT770 Premium models to have similar levels of bass and the 600Ohm model has the most.