I've found exactly the opposite. Why? iTunes have changed the way they handle their audio tracks, since going DRM-free. Instead of ripping from CD, they are sourced directly from the music publisher and the publisher submits the tracks through the iTunes CMS. As these tracks are created from the original sound source, the quality is far far better than old tracks.
Indeed, they are better than those on CD - I've bought various recent albums (Depeche Mode 'Sounds of the Universe' is a good example) and the same version from iTunes. Both myself and a music publishing friend thought the iTunes non-DRM tracks were superior to those on CD. The iTunes tracks were more dynamic.
However.....and this is the big problem. Older tracks will STILL be compressed versions sourced from a CD. Only fairly recent iTunes tracks are sourced directly from the music publisher. So, if you're downloading older albums, the tracks on iTunes will sound worse than those on the original CD.
My advice is to rip your own tracks from albums before iTunes went DRM-free (last year?) and stick with recent albums from iTunes after it went DRM-free.