I'm too good for this world :D
Good info! You can also use the tip of the TRS connector, touch multimeter probes to 1 and to 2 for one channel and to 1 and 3 for the other, as seen in this wikipedia picture. Cable resistance is really negligible.
I strongly recommend to measure HE-500 resistance and
1. If you find imbalance more than 25% between the two channel drivers (which, normally, means more than 1dB power imbalance) ask for a replacement.
2. If balance is fine but you measure more than 30% resistance difference by the nominal value (of 38 Ohm) request for a replacement, also. I have audition a friend’s HE-500 which had almost half of mine’s resistance, and sound quality was really poorer than mine. Of course, if he hadn’t compared it with mine, he could not know the difference because his HE-500 sounded good enough (even compared to his Sennheiser HD-650).
Edited by plin - 6/17/12 at 7:40am