1st ur ear matters: if ur ears cannot differenciate, then anything will do
2nd then the player itself, the chipset, the quality, and so on ....
3rd ur earphone, there is diffrence depends on ur ears
4th ur input file quality, in short the mp3 song itself ... the bitrate, the conversion process, etc ....

conclusion. if u think its good, then its ok. if u think it doenst fit yr ears, means u might need to get a better ones ... no need go for a super expensive ones when u cant differenciate the difference...

my 2 cents ...