Wow, cs600 is better than cs300xs. Unfortunately, he did not mention which part of the sound is 'better'.
Actually, Denon and most headphones have flat impedance curve though. Only the Sennys have weird impedance curve, which can only be 'solved' by constant current amps like Transformer coupled design like what all the Leben models has.
Very true indeed, I stand corrected. But still - I've seen people cherishing the LCD-2/CS-600 while several are not happy at all with the (even lower impedance) Denon D5000/D7000