I had the same problem. M advised me to obtain an XLR to phono adapter from Maplins. Worked perfectly for me. The adapter was inexpensive (I did get it from Maplins if I remember correctly) and you can then run a standard screened phono cable to your sub (usually you only need one - either left or right or the LFE depending on your preference). (Much less expensive than a long XLR cable).

Use your ears rather than your meter to set the sub level would be my only other advice.

Cliff

Main system - Sources (Kodi-HTPC, Vero 4k, FireTV, Sat) into Trinnov Altitude 32 (8-16) (+Roon), Trinnov Amplitude 8 amp & Vivid Audio (Giya G3, V1.5) and REL Stentor III speakers. Others: F80 + Raspberry Pi and other Pi systems.