You take them from your pre/source, you can stick these on the pre’s output or the Belle’s input to get a line level to each of the subs.
https://www.ebay.com.au/itm/1pcs-1-RCA-Male-Plug-To-Dual-2-RCA-Female-Jack-T-Adapter-Connector-Gold-...
https://www.ebay.com.au/itm/Gold-1-RCA-Male-Plug-To-Dual-2-RCA-Female-Jack-Right-Angle-F-Adapter-Con...
Cheers George
https://www.ebay.com.au/itm/1pcs-1-RCA-Male-Plug-To-Dual-2-RCA-Female-Jack-T-Adapter-Connector-Gold-...
https://www.ebay.com.au/itm/Gold-1-RCA-Male-Plug-To-Dual-2-RCA-Female-Jack-Right-Angle-F-Adapter-Con...
Cheers George