Sample files and Purchased file quality

I haven't noticed this. One possible explanation of your Nickleback experience is that the sample is in the middle of the song. During a mix subtle changes take place over the course of a song. For example if the bass is louder it can take away frequencies from the bottom end of the other parts of the song. It could also be that Itunes has been supplied with a radio mix for the single. Generally more time is spent mixing and mastering the single off an album than the other songs. They compress it more to make it louder over all. Radio singles aren't always put on a record (Oops, I mean cd. Now you know, I'm old) because the difference in sound is enough to be noticeable. Take note, that doesn't necessarily mean it will sound better. Just louder. Have you ever noticed how fm rock radio is hard to listen to for an extended period? That is because they are trading sound quality for more gain(volume). If you are really worried about fidelity then you should go buy the cd. If you want better sound quality into your ipod don't convert them to mp4.

except in rare cases, the 30 second preview is taken from the full length AAC file, so they should sound exactly the same.