Do NOT waste your time with the AKG K550s. I had them and was sorely disappointed. I currently have the DT770s and they're great. I recommending going with the higher resistance models (250 ohms), but you will absolutely need an amp to drive them.
If you go with a model with less resistance, you won't need an amp, but I don't recommend that route if you can afford to buy an amp.
I have the FiiO E07K Andes DAC amp and it works great with the DT770 pros. They really fun for kick drums and snare and such. If you're listening to a lot of guitar-driven music (rock or metal), you might actually prefer the Ultrasone PRO 750s. Guitar is more front-and-center with those.
I have both of the cans referenced above, so all of that is based on experience. The only other thing I'd say about the Ultrasones is that I prefer using the earpads from the PRO 900 model - they mellow out the highs a bit.