I too have the Marantz sr 5500 and I think its a great reciever. I don't know what you mean by wanting better stereo sound in music.

If you are listening to music from a DVD or CD player you should use the digital inputs on the receiver seeing that DVD's and CD's are digital.

You should not worry about bi-amping your sr 5500. What I would do if I were you is to bi-wire my speakers.

You have more that enough power to drive 8ohm speaks...How effecient are ytour speakers