Maximum power transfer occurs when the impedance of a device matches the impedance of the source...
Think of it this way: if source impedance is much higher than the device then you risk damage due to overload, like a blown speaker. If source impedance is lower than the device, it is kind of like the resistance of trying to start a 5-speed in second gear...
Your mixer specs list (at the bottom) a headphone output (output source) impedance of 150R (ohms), and I would guess around 100 milliwatts. The DT770 Pro headphones (device) are rated at 250R...
In this case, I think you will be fine because most people don't monitor their music at full volume; so you should have a bit of extra headroom on your headphone output to compensate for the mismatched load. This is my best opinion. If in the end you are unsatisfied you can always purchase an inexpensive headphone amp. :)
Please understand that this idea of mismatched loads can be completely safe because we are dealing with low power, but it can become very dangerous when larger amplifiers are supplying much higher wattage through much lower resistances in speakers. I'm sure you are aware of this because of your bass amp, I just don't want to put the wrong idea out there.
I hope this helps!
