I've been working with the Beyers for many years and have probably owned a dozen different pairs at one time or another. They are excellent headphones. The DT770s, especially the 80 and 250 ohm versions, can benefit from some dynamat mass damping on the inner cups to tighten up the bass, as well as a recable to reduce fatigue during longer listening sessions. The 600 ohm versions are a little better balanced in my humble opinion, with a slightly more prominent midrange and less sibilant highs. You can procure the 600ohm drivers from Beyer USA pretty easily if you'd like to "upgrade". The Crack drives the 600 ohmers without issue.
In addition, there are some unique earpad options out there. Leather pads work well if they are perforated on the inside. They boost the perceived midrange, which I feel is the main weakness of the 770 and 990. Avoid the non-perforated leather pads from Beyer, they add far too much resonance and bass and make things sound bloated (again my humble opinion). In addition, Turbulent makes the leather headbands for some sleeker looks.
As someone else mentioned here, the Denon D2000 / D5000 and D7000 are fabulous headphones, but the impedance doesn't match the Crack. The D2000 is the best value headphone I've ever owned, it can easily compete in the $700 range after recabling and tweaks.