I wouldn't go as far as to say that they're polar opposites,but different, of course.The Akgs sound remarkably similar to the Ety ER6,that I once owned.The sound has more weight with the Hd600,and I think the timbre of the instruments is more realistic with those.The impact of the bass is much,much better with the Hd600,and the highs are about the same,but a tad more sibilant in the Akgs.The Akgs are still very new,and I expect them to change quite a bit.But they are so,so airy and "ethereal"...the sound is very special.They put more distance between you and the music than the Senns,and that's something good or bad depending on the recording.The bass is lean,but very well defined.I can live perfectly with it.The level of detail is in my opinion quite close,but the Senns still win so far.Let's see if I think the same in some weeks...and for the gamers out there,the k501s are VERY good for knowing where the bad guys come from.
All in all,I think that those two cans are equally satisfying and you cannot go wrong with either of them.