Care to share your signal chain, @Rome ?
There are actually two signal chains in my setup:
Guitar -> SY Input -> SY Thru -> KPA input
KPA send -> SY return -> SY main out R -> KPA return
This way the KPA works as usual when the SY is not in use. When I want to play the sounds from the SY-300 then I just add a 'loop distortion' to a rig in the X slot of the KPA. Now the synth and guitar sounds are mixed inside the SY.