Actually Scott, I believe the RS-2's are just a bit smaller than the RS-1's...it is not all that noticeable, but the cups are indeed slightly smaller. B versus C etc.
As for the Omega II versus the HP-2, I own both of them, so I suppose I should field that one. To be honest, it is rather difficult to compare them, as they are totally different types of headphones, and you can't use the same amp to run them. It makes it a bit more difficult. Comparing the Melos maestrobator and HP-2 to the KGSS and the Omega II's, I would say that overall the HP-2's presentation has more energy and neutrality...beats hit harder, every tone is more accurately placed that the Omega II's across the whole range of the music. The Omega II's don't have the bite of the HP-2 and grado, but they have a much wider soundstage and a really effortless presentation. High notes especially have almost instant attack and decay (at least when they are not meant to decay over a long time). Things like cymbals and plucked notes really fare well with the Omega II's. Bassier things like foot stomps, drum beats and bass lines are more satisfying with the HP-2's. In general, the Omega II's lend themselves very well to things like string quartets where you want to hear where each performer is and be able to isolate them. The HP-2's are better for when you want to hear every note at its proper timbre, when you want to listen to rock, blues, electronica or vocals. And of course, each does so well in all categories that these "deficiencies" compared to each other would probably not matter compared to most other phones. That is, both are so good that even where they have weaknesses, they are still stronger than 99% of other headphones.