I listed the DT 770 Pro because it's the model Beyerdynamic bills as their top closed model for the studio. Do you think it should be replaced by the DT 250?
(Tyll measured the DT 770 [600 Ohm], but not the DT 770 Pro, and I'm uncertain as to the sonic differences.)
The DT770 have a more uneven FR curve, but are more of what people think of as the Beyer "house sound" (i.e. bass and treble emphasis). I've met people that like it for monitoring, studio use, and just general use. The DT250-250 sounds pretty different and I wouldn't really consider them substitutes for one another.
The DT250-250 have a more even response curve, a more "neutral" sound as it were. To me, they sound a bit like a closed Sennheiser HD580/600. Tyll already pitched the NAD Viso HP50. (And I'd second that too). I think the two compliment each other very well. The HP50 has much better quality, far better extended bass and overall better imaging. The DT250-250's bass rolls off quite early and isn't nearly as forcefully present. On the other hand, the DT250-250 have very good detail and aren't as warm as the HP50.
IMO the strength of the DT250-250 (like the Sennheiser HD600) is that they never, ever, ever, ever cause listening fatigue. Ever. The mild bass emphasis and (quite excellent) impact on the NAD can tire you out on marathon listening sessions. Cosy at both ends - the DT250-250 are a real good workhorse. As a bonus, I could probably throw them out my third floor window and the DT250's wouldn't break.
Speaking of things that won't break even if I parked a mid-sized sedan on them: I'd also second the earlier vote on the HD 25-1 II/HD-25 Amperior/HD 25 Aluminum (and presumably the HD26) line. I have two pairs of the Amperior (in silver and blue) and if you can tolerate their clamp and don't are fine with On Ear headphones, these are also spectacular.
Extra Note: I'd take what companies bill as their "top" model with a grain of salt. Of course a company is going to market their latest product as the best thing since vegemite on toast. Notice how no recent Denon models have popped into the conversation yet? I'm sure that they have some to market as excellent professional headphones, but marketing sure doesn't make it so!