Hi John,
Haven't yet seen a SX900 (mine is a Genos) so they may not operate in the same way but maybe this will help.
On my Genos, I press a button marked VOICE and this takes me to a screen with all the chosen voices shown complete with all settings relevant to each.
I can then press OCTAVE for any one of the four voices shown and raise or lower by up to 2 octaves without altering any of the other 3 voices.

You used your Technics in exactly the same way as I used to and my KN7000 is still my favourite keyboard of all time.
Tony