I am fairly certain they are the same internally, but with different exterior styling & cables (Pro = coiled; Limited & Premium = straight). Here is some additional info that I came across:
http://homestudiobasics.com/beyerdynamic-dt-990-pro-vs-premium-end-all-confusion/
I have the 250 Pro model & I agree with XERO1: They are some of the best sounding under-$200 headphones I've ever heard.
Thanks for that link. I've been thinking about getting an open back hp to complement my Senn. Hd5x8's.I want a pair of open backs that are more v-shaped with more bass than the senns.A more fun sound. Don't get me wrong, I like the senns neutrlalish sound; but for a change of pace, with some music I want a more fun sound. The Dt990's appear to be what I'm looking for. The only thing that concerns me, is that the treble might be too much for me. Although I could equalize the treble, to tone down the highs, if they're too much.
I've got a line on a used pair of Premiums. Think I'll buy them.