Need a Volume adj knob for line outs with the URRT4!

Hi Need a Volume adj knob for line outs with the URRT4!!!
LINE OUT 1+2 goto MON 2 USB / Aventone Mono Speaker
LINE OUT 3+4 goto RNHP headphone amp with Vol knob
MAIN out 1+2 goto Hafler 1500 amp then NS10 speakers (Vol Level on front)

Looking for a way to not have to readjust levels everytime I listen to a track in Reaper with URRT4 setup vs listening to internet audio. Leaving the URRT4 on, going to internet audio (PC to USB to Mon 2 USB to Aventone) levels are hardly there at all.

If I have the Asio URRT4 in Reaper +0.00 dB output and as the Default Device and Default Communication Device in the MSFT Sound Module WIN 10, then I can set the KNOB on the MON 2 USP @ 9am with Meters in Reaper at -12 to -6 with decent not loud audio matching internet Youtube Music Audio Levels going through the Line UR-RT4 Device with the MSFT PC slider audio level on the pc set to 100.