The "Pro" versions of dt990 are only supposed to be available @ 250Ω, with coiled cabling........and still the without changes from the 2003 version. If you want the best-sounding available version, you really want the 2005 600 ohm version from the consumer line. There is a big BUT to this recommendation - you will need a good solid strong amp............I've been using a Corda Prehead MkII with mine - and there are times I wish for a bit more. A primary value to getting this version is the smoothness of the treble........there are recordings where the 250 ohm version where the treble becomes too much and you want to use the alternative 120ohm outlet on the Cordas, or turn down the volume a bit for the entire recording (Star Wars would be an example). ..........on the other hand, the flip side to all this is you would be buying one of the fines headphones on the current market.
Another note - generally, the 32 ohm and 600 ohm versions are only available thru the Beyerdynamic Manufacture page (tho' it is said there are one or two dealers in either Singapore, or H.K. (Jan Meier used to have a stock of 32 ohm version - but not any longer).
In terms of running dt880/600 - My experience differs from Trose49 above. It seems amazing, but true - the Xin Supermacro LE does a very nice job running those particular 'phones. It is only a comment about this particular pairing and not other models or manufacturers. Plenty of headroom, and never any clipping.