Just a note on the Cello, it was not intended to address room anomalies, it was intended to compensate for the equalization and gain-riding used by record companies in mixing their recordings to make them more listenable on a good system; that does indeed translate to "musical", and it is a great product in that regard. That is the other use for an equalizer in your system. Jameswei's post is excellent; I'd only add that many beginners wind up using an equalizer ultimately to add overall gain to the system; I know when I started out that's what I did. Don't fall into that trap, as all equalizers, at least those in the analog domain, do degrade the signal to some extent, so it's best to use equalization sparingly to affect only the frequency ranges that help you enjoy the music most.
