If I wanted to expand my music, I would do it not too radically, i.e. I would buy albums from artists which have high similarity to the ones I like.
I would advise Scrobbler to link an iTunes playlist and a Last.fm account. It recommends some great artists and is the best artificial recommender I have come across.
Friends have also been a big help but they like very similar artists.
Here are my recommendations to start off:
Has yours come through yet?