Hi. To check hp out, you can use any normal stereo trs to 2 x mono ts cable and plug in a amp or mixer to check the level without having the relative hi load (low hp impedance) affecting the sound.
On the unit-out page, phons parameter, you can select 7+8! which will give the same fixed line level as 1 to 6.
Maybe called 5+6! since di3 has 6 outs.
(The out12 par can also be at fixed level with 1+2! )
This way you know if the hp out is all bad or just cant drive a lo imp load.