line level

I could use some wisdom, please. I have been running a small patio 2.1 setup for 3+ years and it's been great. Sonos Amp powering a pair of Polk Atrium 6 and then sub out from Sonos to an old Polk SWA500 subwoofer amp and the polk amp wired (4 conductor) to a Polk Atrium Sub100 (dual 8ohm voice...

I have a MONO LINE LEVEL OUTPUT and I want to share it with 2 devices (which will received the LINE LEVEL OUTPUT and use it for whatever reason). In one of those devices I need to use a 10K potentiometer in order to control the signal level that goes to one of those devices. See my drawing...