I am not sure what setting you're tinkering with there. Go to DSP Program/ Surround Decoder/ Pick DSU and set center spread to 'on'. Don't worry too much about tweaking with custom soundfields yet ( i assume you have a Yamaha Aventage unit?).
If you have a smaller music venue in your town, perhaps a bit bigger than a larger home listening room. Go there and try watching some unplugged band, for instance. I recently saw a couple of string quartets sitting up close. It sounds nothing...and i repeat nothing like stereo. It sounds very much like Atmos to me!