the differences between the premium and pro are not many. heres a list:
-premium has a bit less clamp to the headband
-premium is a bit more airy sounding but has less bass
-premium has slightly different earcup cover design for aesthetics
-premium is available in 32/250/600 not just 250 like the pro
thats about it. the 250ohm pro is going to sound about equal to the 250ohm premium (other than less bass more airy as described above) though it will sound better than a 32ohm premium. keep in mind that just because it is 32ohm does not mean that it can be driven by just anything: while it is certainly easier to drive than 250ohm cans it isnt "easy" to drive.
if you went with premiums, i'd get the 600's and a nice amp to make it worthwhile. otherwise, i'd stay with the pro since they are more for your money. your choice of course.
i think you pretty much settled on the dt990's then?
while its not "entirely" accurate i'd listen to some sample tracks on
http://www.sonicsense.com/resourcecenter/app/audio?preset=over_ear_headphones&source=result&track=beyerdynamic_dt990600 to compare the different headphones. its not going to sound "exactly" as they will on your current speakers/headphones but will let you see how sets compare with one another. specifically note the headphones listed above.