I used to use the ambiance-extraction feature in an old JVC DSP unit. If I understand it correctly, it just takes the L-R signal and routes it to the surround speakers, capturing what I understand to be the natural ambiance in stereo recordings. I listen to classical music, and I found that this approach can add a very satisfying sense of concert-hall ambiance, which is one of the things that is seriously missing in home reproduction. I don't know how well it works for studio-produced music, but I enjoyed the naturalness of the approach on many recordings, and it's got me thinking seriously of giving the Syn a try.