I've noticed a lot of people calling the c5 bright. Personally, I have to stress that it isn't bright. It's very flat. There are many factors that affect how it sounds with different earphones, such as impedance. What is most likely the case is that the last amp someone had was a different impedance causing attenuation. Now they're hear their earphones as they should sound. With a <2 ohm output it's very accurate to an earphone's intended sound based on impedance.
Just saying, take everyone's opinions, but realize the sound they hear is a result of many factors and not really indicative that the amp itself is bright or bassy or otherwise. The c5 has a very very flat response. :-)
If the O2 is considered neutral, as it is my reference amp of choice, then yes I find the C5 to sound be a bit brighter than neutral. That's been the case regardless of the headphones/earphones I've tried.
Of course the C5 measured to have a flat frequency response curve, most amps do nowadays, but factors other than the frequency response are responsible for an amp's sound.