I used to drive my fa011 directly from my pc (creative audigy2 and kx drivers). the sound level was adequate.
Today I have got my O2 (I just could test them for an hour because it’s a xmas present) and tried them with the FA011, the first impression was mainly that the fa011 could produce high tones. the bass hasn't changed at all, its sounds actually less because of the more higher tones. I am a little bit disappointed because of this fact. Mainly because in this thread was suggested that FA011 amped would be better with sub bass (with is not the case, maybe this is true with other amps? Amps that are colored )
But the upside of the more higher tones is that the fa011 sound wider.
As an example http://soundcloud.com/bopiliqod/nsdfs the rain, crow and thunder sounds a lot better.
It depends a lot with the amp, the O2 is not a coloured amp which is almost flat lined neutral, technically given that it only add's a gain to your music without affecting the sound characteristics much, if you used something else different perspectives of the sound will be enhanced or boasted.