Since I live in a country where buying Dream Theater music on CD would cost me a fortune, I have resorted to buying their music through online download. With D/T, I bought the music through the iTunes store for the first time, after deciding to ditch HDTracks because it eats so much space in my phone. I liked its quality, and I was actually confused when I read comments complaining about the quality of the audio of those who listened to the CD.
Then I noticed the label "Mastered for iTunes". Read up on it and was surprised that music with this label uses a different master from the CD. It is specifically mastered for a portable format like the m4a files of iTunes, not just downsample it from the CD master.
Curious, I bought the "Mastered for iTunes" self-titled even though I already bought it before in mp3 and HDtracks. And I was surprised when I listened to it. It does sound better than my old mp3s, as I can hear the cymbals clearer and some of the clipping was reduced. I listened to the two versions one after the other and there really is a difference.
I'm thinking of rebuying the other albums that are Mastered for iTunes (ADTOE and TA) as my old HDTracks are in my old laptop that I have no desire to reopen anytime soon.