I've compared the DT770 Premium 250Ω and 600Ω, and they were a clear night and day...much clearer trebles and much faster response. Hell, they even were much lighter when having a 250Ω driver in one hand and a 600Ω in the other(I bought the 600Ω drivers as spare parts).
"The impedance is determined by the voice coil (dynamic headphones), which is a winded copper wire (coated to avoid a short-circuit). This copper wire is available in nearly every length, but not in every gauge (thickness) and a thicker wire has less resistance than a thin wire ("less fits through"). The magnetic field of the voice coil depends on the number of windings of the coil, causing a low impedance system to use a thicker (also heavier) wire and since the membrane foil can't be infinitely light-weight, the moving mass (voice coil and membrane) is relatively high. It's pretty clear that a higher mass can't move as easily (following an audio signal) as a lower mass. This low mass can easily be accomplished with thinner (lower weight) wire, but the thinner wire has a higher impedance. This means that the DT 770 PRO with 250 ohms sound more natural, but plays (depending on the used headphone amplifier) not as loud as the 80 ohms version."
being skeptic is good, but these are FACTS. And yes, my 32Ω cd3k kills all their 600Ω Manufaktur phones but that's just how BD engineer their drivers...higher impedance = faster reponse and clearer trebles, just like nikchen said: http://www.head-fi.org/forum/thread/424842/beyerdynamic-dt880-closed-back
"The 600ohm driver have much more precise bass and the overall sound is clearer. These are really a big step and should be preferred. Of course the result is good with the 250ohm systems but you give away potential."