That doesn't make much sense. I use my O2 as a desktop amp also. Clipping the 2.5x would have given me unity gain for my ultra-sensitives and left me the option of playing from my mp3 player or HD650s (serious desktop headphones).
I've never seen a justification for clipping that 6.5x that adds up.
For some reason people have got this idea of clipping the 6.5x in to their heads (justified by the higher distortion at 6.5x) and encouraged others to do so too, but all it does is limit your options. That higher distortion isn't audible and is irrelevant.
Justification for clipping 6.5x gain that adds up:
1. You do not intend to use low voltage sources (<=1V) since your dac/soundcard outputs 2V+
2. You don't need gain for sensitive headphones, but enough voltage swing for higher z headphones
If you remove the 2.5X option you limit your voltage swing or risk clipping the input.