Hi Dave,
So a simple set up like we have with piano ( controller) / sx 900 would be a good way for John to go with the Pa4x?

Hi John
Just thinking if Pa4x was only used as controller like I do with my p121,
Registrations set up for Sx900 can still be used probably as is.
Better key bed on the Pa4x,
I forgot about PA 4x transmitting aftertouch ,Sx900 doesn’t have it,
but from a earlier posts on the forum,it appears Sx900 can receive it,
that alone for me would make it worth using Pa4x as controller for Sx900.

I basically , with my “simple setup,”
I can play Sx900 independently from the p121. ( or Pa4x)
I can play the p121 ( or Pa4x) independently of the Sx900.
I can use sx key bed for left hand chords and use the whole p121 ( Pa4x) keybed for playing r/ h melody. ( basically independent of each other)

I can use p121 ( Pa4x) as controller for sx 900 ie
l/h chords and r/h melody are played on the p121. ( Pa4x)

So might be worth checking into John.