Thats probably because the advertised 18mW/channel output of the M6 is the peak output and not the RMS (average) output.
You'll get distortion with sound enhancements with the M6 because thats just the way it is.

Loud volumes + bass enhancements in particular will cause distortion in just about every song you have.
Impedence of your phones' will play a part in how loud your miniPlayer can get. Generally earphones with lower impedence (~16ohms) will be louder than a set with an impedence of 32 ohms. Of course, don't try anything lower than 8ohms (aka plugging in power hungry speakers into your miniPlayer), as they will draw lots of current from your m6's amplification circuit and damage it. The effects of course won't be apparent immediately, but you'll run the risk of shortening the life of the player