Thanks for the info everyone.   I do hear a switch/click when I change from normal gain to high gain.

I have:   Creative Sound Blaster ZxR Sennheiser hd600   In the driver, it allows me to choose Normal Gain (32/300 ohm headphones) and High Gain (600 ohm headphones).    Which should I choose? (it says "Setting to high gain may damage your headphones if their impedance does not...