Hi,
i use a Studio 68c and the HP4 Headphone Amp.
On Line 1/2 of my 68c i connect my smartphone via a Y-Cable TS Mono to Jack on the other side for my smartphone.
Then use 2 Mono Cable to connect from Monitor-Outputs of the Presonus to the Input of the HP4, where my Headphone is connected.
Turn on Line Level 1/2 half.
Turn on Line-Button on the front.
Turn on Main Level half.
Now got to Sound Settings on your PC and go to tab "Recording". Select your Presonus Line 1/2. Right click into the Settings and select "Listen" where i turned on "Listen to that device" ... 2 correct signal (left/right) ... Perfect.
Thats my workaround.