E-Mu XK6Xtreme Keys Owner's Manual page 27

Therefore, the Sub 1 jacks can serve as effect sends and returns
in order to further process selected instruments and then
return them to the main mix.
You can use the Sub 1 jacks as send/returns in order to further process
selected XK-6 presets without using the effects bus on the mixing board. In
a pinch, the effect returns can be used to sum additional instruments into
the main outputs. It's like having an extra line mixer when you need more
inputs!
Output Section
R Bus
Tip
You can use the Sub 1 jacks as effect returns to the Main Outputs.
Note that the Effects Processors are only routed to the Main Outputs.
Processors
L Bus
Tip
Ring Ring
R L
SUB 1
Studio Setup
Effects
R L
MAINS
XK-6 Owners Manual 17
Setup