When the song was compressed to mp3 initially, a lot of detail was lost which is how the file size got smaller. Now that detail is lost forever and cannot be regained. Some softwares claim to regain lost compression details but most often they just boost certain frequencies.
Now when you convert that mp3 file to another format, unless the conversion is lossless, you are going to LOSE EVEN MORE details. So no matter what format you use, whether it is aac or ogg, it'll be worse than the previous version.
aac and ogg are superior to mp3 only when you directly compress the original source, say, an audio cd to these formats. But if you convert an already compressed mp3 to these formats you won't get better quality. In fact it gets worse. So just let it be in the mp3 format.
As for the CX300 allow them to break-in. Some speakers need to be broken-in for them to perform optimally. Connect the earphones to a source, turn the volume upto 80% and let them play for some time continuously (like an hour or two). See if there is any difference in the sound. The bass will definitely be deeper as the drivers will lose their initial stiffness and will move more freely, thus moving more air.
Btw, how much did you get the CX300 for?
