haven't tried the C4, but the C3 is very affordable with better SQ than sansa and ipod. But i've seen many preferring the DX100 than the AK120 though, but i won't comment on them since i haven't tried the DX100.
The AK100 output impedance is around 22? Not suited for most IEMs as they are around 16, but then again numbers doesn't mean anything
FiiO X3 is almost comparable to AK100 and less than 3x the price which is really reasonable, but build quality and aesthetics doesn't come close to the AK100. So tempted on the AK100 just on the basis of looks lol 

22 Ohms Output Impedance

What is this?:

Almost just almost this player came in nearly flawless but it has a problem in the amp section that is perhaps unforgivable. The output impedance of the amplifier stage is 22 ohms and this is the cause of some very serious problems. Due to the effects of damping it is a rule of thumb that your earphone should have an impedance of roughly 8x the output impedance of the player, meaning ideally this should not be used with anything with impedance lower than 176 ohms. An effect of this is an obvious drop off of bass, reducing some headphones to something that is completely bass-less.

You also have the effects on frequency response, because a lot of headphones do not have flat impedance plots, especially multi armature IEMs with crossovers that can be quite all over the place, the high output impedance coming from the AK100 means less voltage enters the earphones and this makes the different frequencies get a significant amount of difference in volume resulting in a obvious colouration and screwed up frequency response. It is noticeably different and you will be able to tell what the measurements confirm.