What levels are you referring to?
What mix are the headphones listening to? They can be set to any output mix, so set to main LR as a starting point.
Whatβs the headphone model and input impedance?
Mine drive headphones very well.
As an FYI, my headphone ports on my 18T don’t work either. Did all the troubleshooting, headphones work on guitar amp, tried ‘listening’ and ‘Main L/R’, no channels were solo. Reached out to the US distributor and they are going to RMA the unit after a factory reset didn’t work. I found a guy on another facebook group with the same problem on a 12. He RMA’d also.
There may be a build QA issue here. The mixer is working well otherwise.