To expand on this (and repeat what I just posted at NPR), Death Magnetic sounds worse on CD because of dynamic compression, not digital compression. If digital compression was the problem, then the Rock Band version would sound WORSE than the CD, because the CD has no digital compression (and Rock Band does). The CD is what you make MP3s out of, after all.

The CD master of Death Magnetic has had its dynamic range heavily compressed (which squashes the waveform), where the Rock Band version is uncompressed because the game mixes the song on the fly.