Turns out Native Instrument's new Kontrol S4 MK2 has a hidden gem in the new update: Booth Out!

According to the new User Manual, on the back of the S4 MK2 are RCAs for Main Out and 1/4 for Booth Out. There's a recessed Booth Volume knob next to the 1/4 outputs. Next to that knob is a switch for Booth / Main. If set to MAIN the 1/4 outputs will output same as RCA. If the switch is set to BOOTH, the 1/4 output volume only uses the back Booth Volume knob and is independent from the Main Volume knob.

Since most pros don't fiddle with main out volume, the easy work-around is to swap roles and use 1/4 for main out and RCA for booth. You can set a good main-out level using the back knob, and use the convenient main volume knob for your booth.