UR44c - difference in main/line output level?

Hello dear friends and greetings from Greece!

I am a very satisfied user of the UR44c interface and didn’t have any problems till now (apart from the tricky installation). I connected today the line outputs 1R 1L to an external console, mainly because I wanted some more volume for tracking with my DT770’s (250 Ohm). I immediately realised that the mixer (Yamaha MG06) was peaking all the time and had to use the first two inputs, engage the pad and turn the gain to zero on both channels so as not to get red lights and distortion. The same was happening with outputs 2R 2L. Then, I unplugged my monitors from UR’s main output and I connected the main output to my mixer in which situation I had a perfect signal, clean and crisp, even with the output knob to the maximum. Can somebody please explain this to me and also tell me if there is a possible fix to this? I mean if I can somehow lower the output of the line outputs of the interface? Thank you in advance. As I said, I’m by all other means totally satisfied by the interface, used it on an i3 with Windows 10 till now and three days ago I bought an M1 Mac mini installed it and everything works great, both the interface and Cubase 11. This use of the additional outputs would just help me a lot more.