Mastering engineer Ian Shepard does a null test1 on three versions I’m With You by the Red Hot Chili Peppers. This was the album that had a specific master for iTunes.
Go to the link for the Youtube video. Since it’s youtube you won’t really hear the exact audio, but you’ll get the point.
His findings have more to do with that iTunes Master for the RHCP record than it does with Apple’s Guidelines for iTunes Mastering. iTunes Mastering is not aiming to sound like a CD. It’s aiming to sound like the original source material. But in the RHCP example, they attempted to compensate for the encoding.
A Null Test is when you take two waveforms, invert one, and play them at the same time. If they’re exactly the same you should hear silence. If not, you hear the leftovers. I’m probably butchering this explanation. ↩