@discnik
Maybe you can try connect the sub to 'MAIN IN' instead of 'PRE OUT' while the switch is at 'NORMAL' position. If no signal output to the sub, next thing to try is using a 'Y' adapter connect one end to 'PRE OUT' and the other end to 'MAIN IN' to loop the signal back to power amp section. The sub connect to RCA female end and set the switch to 'SEPARATE'.
https://www.amazon.com/Adapter-Female-Splitter-Amplifier-Subwoofer/dp/B07LB5CCTB
Maybe you can try connect the sub to 'MAIN IN' instead of 'PRE OUT' while the switch is at 'NORMAL' position. If no signal output to the sub, next thing to try is using a 'Y' adapter connect one end to 'PRE OUT' and the other end to 'MAIN IN' to loop the signal back to power amp section. The sub connect to RCA female end and set the switch to 'SEPARATE'.
https://www.amazon.com/Adapter-Female-Splitter-Amplifier-Subwoofer/dp/B07LB5CCTB