Bernie at least your trying different configurations to suit your needs, and mixing it up as it should be ....
sorry your not there yet but close...let's face it the SX900 is mainly an
arranger kb even though it has a few other features....
but it's main use is using it as an arranger as it should be
and does that well imo..
why not just use your Korg Pa4x 76 which I would say it's features are
more multifuntional friendly for arranger/tracks smf cdg,play/etc,...
wouldn't that come close to your perfect unit handling
all your are asking above...?

good luck