For me with JH-16 I use 25/40 on the i9, and the Slim volume knob at 50% max.
On my i9 alone unamped, the JH-16 can run at only 10/40 volume, but for the same volume, my Westone3 would need 12/40. I think the JH-16 are easy to power, but needs a good amp to actually bring out its true sound quality!
What IEM are you running on?