Although I haven't tested this on the Tyros's, in GENERAL, the line-outs of most audio equipment is a hair cleaner and quieter than the headphone outs, as it doesn't have to pass through an additional amplifier (to get it up to phones power levels), and built-in headphone amps generally don't stack up to dedicated pre-amps.. Fewer gain stages usually equates to quieter, cleaner sound.

Just a thought...