If you have autocalibration, it should set the sub level. If not, use a SPL meter and set it to 72-75 db. The speaker should also be at 75 db at the listening position. Connect the RCA cord to the left sub input or use a Y connector and connected to both inputs. This may get you an extra few decibles. I deally have the sub close to the front speakers so that you do not hear the sub as a separate speaker. You should not be able to localize the sub.
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus 200 Five, Dayton 18 Ultimxa Dual Sub Cab(2), Dayton 18 Ultimax Large Vented Sub Cab (2), on Berhinger I Nuke DPS amps, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500