About RCA XLR audio output of DAC, which is often misunderstood
The RCA and XLR outputs are basically voltage outputs and are not capable of powering headphones or the like. The power W is the output voltage V multiplied by the output current I, and is calculated by W = V x I.
The RCA or XLR audio output can output a sufficient voltage V, but has almost no ability to flow a current I. So it doesn't have the power to drive headphones. Forcibly connecting the headphones may damage the audio output circuit. The output impedance is quite high, but in rare cases it can work well with high impedance headphones or low volume.
Even with the same audio output, the headphone output constitutes a small power amplifier that has significantly increased the current I capacity.
In other words, you are saying that since V = I x R, and voltage is fixed to 2-4V, the output current I of DAC is typically too low due to high impedance (resistance)? I believe this is generally the case, despite no info about M300. My experience seems positive with my headphones, but I do not want to risk them any further. The reason I tried M300 with headphones is that apparently the headphone line out of my (vintage) Denon 720PMA amplifier is not on par with the speaker output. The sound from the headphones became very altered in the tonality with boomy bass. This is not the case at all with the speakers (wharfedale diamond 200).