Equalizing the Volume

About 80% of my listening these days is computer based--streaming from my mac mini to squeezebox to dac. My digital collection, like most, contains cd's recorded at varying volumes depending on the way they were mastered (among other reasons). This is particularly frustrating when playing mixes for dinner parties, etc. I'm wondering if anyone can recommend software that equalizes playback volume of hard drive-based music without unduly degrading the sound quality. Many thanks for your help!
Itunes has a feature called sound check (go to:PREFERENCES:PLAYBACK), and click it on. That will help you a bit. If not, you can adjust the playback level for each album in the get info menu. Choose an entire album and right click to get info, go to options and on top will be the playback volume. I've spend numerous hours doing this, tiresome, but effective.