Best Classical to "broaden" listening arena?

And the plan is?

Since I really appreciate individual instruments & have a fairly high end system with separation as a strong point--I am interested in which composer / group I should start my "classical" specific collection with?

Your post indicates you want to show off or test your system as opposed to learning to appreciate classical music. What does the price of your system i.e. 'high-end' have to with getting involved with classical music? However if you are serious, start with the tired and true. Stay away from all the twentieth century stuff until you listen to the Beethoven, Mozart, Bach, Brahms and all the other major germans. You could spend the rest of your life with them. Don't try to do it all at once. Subscribe to either or all of: Classical fm, BBC Music and Gramophone magazines. Great reviews, and they give a lot of history which makes the music even more enjoyable. Other than me, don't listen to anyone Audiogon! And buy used on amazon.

To broaden my musical tastes I would buy good condition classical music records from the local thrift shops. It took some digging but ended up with a lot of Tchaikovsky, Mozart, Bach and other artists. The records were so inexpensive and in very good condition. Most I liked and some I didn't. The ones I didn't like would be dropped off at a totally different thrift store then where I originally purchased from.

There are some great suggestions here. I would recommend Vivaldi's Four Seasons by Sarah Chang and any number of Aaron Copland's works performed by Leonard Bernstein. These can be found used on Amazon really cheap.