Hello, "low" or "normal" (medium) gain objective 2 model for 60 ohm headphones?

Whats your dac / source?

Low and medium both have 1x gain as their low setting, but medium has 3.3x vs 2.5x on the low model, no there's not real any reason to get the low model because medium can do everything low can do.
 
Standard is 2.5x and 6.5x so it is the safe bet if you are worried at all about having enough gain. Also with the standard you can easily remove the low gain resistors to convert it to 1.0x and 6.5x, giving the biggest range which will work with just about any sensitive or power hungry headphone.
 
The impedance alone is not enough to say how much power or gain the headphone needs. For example, the Hifiman HE1000 and AudioTechnica M50x have similar impedance at 35 and 38 Ohms respectively, but the HE1000 needs 7 times more voltage to reach the same loudness.

Thanks for response, the source will be onkyo 7030 cd player (supposed to have good dac).

Thanks for the response minds mirror very informative.

The datasheet of your CD player states 2 Vrms on the analog out. So Id go with gain settings of 1.0x/3.5x.

Or you could go 3.5x as low gain and then choose your high gain setting depending on wether you wanna hook up other devices to your O2 with less than 2 Vrms out, say an iPod or whatever.

Boy you did my homework on that. Thanks alynx!