When you say”directly from the mix outputs” I assume you mean connecting headphones or ear buds directly to the XLR output with some kind of an adapter.
As Steffen mentioned that may not drive them with the output you need. Headphones and earbuds come in different impedance ratings that would have some bearing on the level you would get.
Now for the adapter to even try it……
The XLR is a balanced line output,pin 1 is ground, pin 2 is positive and pin 3 is negative, positive and negative as to the phase of the output signal. If you would connect the headphones left ear (normally the tip on a headphone plug) to say pin 2 and the right ear (normally the middle ring on a headphone plug) to say pin 3 and then the
ground to (the larger sleeve section on the headphone plug) to pin 1 you would get a very pronounced cancellation of the signal with a vacuum feeling in your head!
You would want to make up an adapter that uses XLR pin 2 and Pin 1, connecting both head phone channels to pin 2, that would cut the headphone impedance in half as seen
by the XLR output possibly lowering the output. You could try stereo by using two outputs with each pin 2 of one connected to left and pin 2 of the other connected to right, ground to pin 1 on one of the outputs.
Personally I would get one of the many battery powered headphone amps and call it good.
