hi AkA,
By coincidence, I happen to have the same brand of keyboard, although I suspect that your md1700 has more keys than my optimus md1200.
That's right. The keyboard does not have a line out. It has a headphone out which works quite fine. THe sustain input is just to add a sustain or sustenuto pedal.
I'll send you a demo of a song I did with the md1200. The headphone outs are good, except that they are slightly noicier than line out, but its not something anybody but the song creator will notice cos I have heard more noise from pro recordings.
By the way, I love the fingered bass sample on the md keyboards