djwhog
You'll probably be fine using which ever setting sounds the best to you. I don't think either setting would cause damage to the amp or headphones. Yes, it is technically better to use the setting that gives the best damping factor (low output impedance setting); but it doesn't necessarily sound the best.
My example of this is that my headphone amp has a high or low gain switch which also changes the output impedance respectively. Many of my headphones are under 100 ohms; but I almost exclusively use the high gain setting. Yes, the bass does tighten up a bit on the low setting (because of the higher damping factor
) but the high frequency extension and sparkle is diminished and overall the music sounds dynamically flat and lifeless. I even live with the slight hiss in the background and the small range of volume adjustability because music just sounds better on the high setting.