520- I've done alot of work myself on my sound over the past year or so. I sound completely different now than I did before.
My development began with re-thinking my concept of sound. You are already 2/3 of the way there by saying "rich", instead of "dark".
I did alot of long tones, scale work, mouthpiece buzzing, leadpipe buzzing and work on the lyric solos in the Arban "Art of Phrasing" studies in the back of your Arban book.
Additionally, alot of breathing work has taken place. Check out "The Breathing Gym". (Someone mentioned it on here not too long ago).
Look in Manny's forum for his "game". He posts one every so often...I think we may be due again (hint, hint
). Basically, it's a chromatic scale excercise that opens up your sound in the middle register.