I've been enjoying my HD540s for a few weeks now. They are quite transparent sounding cans, but to my ears they have a tizziness and slightly unnatural character to the treble in general. It's not really bad, just just enough to notice that it's a definite colouration of the headphone lending itself to everything I play through them. I do enjoy them though but I couldn't elevate them to the status that some have done on this thread. I'm using the hifiman velours and playing mostly through an HRT Microstreamer or HRT>Cayin C5.C
Could be the hifiman earpads. Source and amp also could be it i recommend graham slee amp's like graham slee bitzie is a excellent companion with hd540.
Also give the hd540 alot of burn in time. hd540 ref 1 600 ohm is little smoother than hd540 ref 2.