If I were to describe the signature of the ref5, I would say it has none, I got lots of different players and switching back ond forth between the players is great fun, because the ref5 delivers what it is being fed no more no less.
Lots og phones have a tendency to show their signature no matter what your source is (Grados will never sound like senns), the ref5 is in that regard a true chameleon it simply changes it signature depending on music, player, amp, type of file and so on..
If one could make a blindtest of these things, I would honestly believe that it was two different phones, when somebody changes source (I can not do that with blindfolds
)..
Regarding the weaknesses of them, well to me there is none, some would say that the soundstage is to small, but phones with the big stage like AKG 701 and DT880 always sound artificial to me, popmusic, rock, hiphop etc is not recorded in a concerthall like classical music or opera is, so the music of the "popular" genres really has no soundstage as all the instruments and singing is recorded in a studio separatly and are afterwards mastered to make a coherent sound, but a real "Stage" is not present..
When listening to classical music you might not feel you are at the Madison Garden, but at a smaller venue, the separation of the instruments is clear and decise but they do not spread out on a stage 100 meters away and 50 meters wide as when listening to AKG701, more 10 meters away in a normal concerthall..
Hope you can use my comments..