Change volume range for sound? (i.e. dB-range for slider)
I also had this Problem a years ago by then I found no way to do it with OS X Onboard Tools didn't even try nor know the AppleScript Workaround (this would have been enough for me i think) anyway.

My Solution I ended Up using was to use Soundflower https://rogueamoeba.com/freebies/soundflower/ a Software based 2 or 64 Channel Audiorouting Tool.

Maybe it also had some inpact on my decision to use it that I already used it for Music Production.

I think the Way I used it to do what you're looking for was as follows:

Soundflower shows up as 2 Separate Audio Devices on the Audio-Midi-Setup a 2Channel and a 64Channel Device both with individual Soundlevel Sliders.

So I just set the System Out under Systemsettings to Soundflower 2Channel and then set a Audio Level on the Soundflower 2Channel Device.

By that time Soundflower was a free Tool don't know if its still free and also am not 100% shure if I really only used Soundflower back then or together with Jack http://www.jackosx.com.

As I said it was years ago and I can't remember all exactly also I now have different Audio Setup much more complex so i'm currently not able to try it out by myself.

Hope it still helps you.