*By the way: I must say that, from what I've heard (DT990 and SR325i using Mahogany Orenholz cups -- Also have V4 Magnums but I won't consider them as "Grado's" for this), both Grado and Beyerdynamic have a quite similar sound "signature" (at least with the two headphones I've heard). Both tend to be bright, have "punchy" (bot really "deep", extended) fast and precise bass recessed uper-midrange and emphasis on high frequencies.
DT990 has a more "grainy" noise floor (not really silent) and show up a little bit more microdetail, also it has a far better/larger soundstage and tend to be less "analog-like" sounding than the SR325i, both are quite "U" curved in frequency response (the DT990 having a tad more midrange). Other than that they are, to me, really quite close in terms of sound signature, even though they have some obvious differences I still think they can be interpreted as having the same "type/kind/style" of sound (but one, the DT990, presenting that style in a more analytical way and the other, SR325i, presenting that same style in a more tube-like/analog way).