Definitely, DO NOT put a jumper between the left and right of the main in.
Remove the left and right jumpers, then get 2 (one for left and one for right) RCA Y adapters like this (2 male to one female)
https://www.audioadvisor.com/prodinfo.asp?number=AQFLX-XFM
Use one adapter for each channel connecting the pre-out/main in. This just replaces the jumper that was removed giving a spare female connection for each channel. Then connect that spare L&R to the subwoofer’s L&R RCA input.
Remove the left and right jumpers, then get 2 (one for left and one for right) RCA Y adapters like this (2 male to one female)
https://www.audioadvisor.com/prodinfo.asp?number=AQFLX-XFM
Use one adapter for each channel connecting the pre-out/main in. This just replaces the jumper that was removed giving a spare female connection for each channel. Then connect that spare L&R to the subwoofer’s L&R RCA input.