Headphones need power so that the drivers in them can produce music. More inefficient and/or higher-impedance headphones require more power to properly control and move the driver to its full potential.
I tried :#
You did very well, thanks!
So, the O2 may not be ideal for, say the 300 Ohm Sennheiser HD 650, but pretty perfect for any low impedance IEM. Well, I just love the sound of the O2. my mini review here.