Some indications (external audio mode test on)
The Rog 5 is delivered in France with intra headphones (pedestrian kit) but I don't know the characteristics, I measure 80dB (A), which is a bit low.
2 non-intra headphones (pedestrian kits), 4 and 8 ohm, 90 dB (A)
Sony HIFI headphones, 220 ohm 85 dB (A), (jack 3.5)
BOSE QuietComfort 35 II headphones (Bluetooth) the volume is 90 dB (A),
Same BOSE headphones (connected Jack 2.5-3.5) Unknown Ohm, volume is 90 dB (A)
The Rog 5 does send a different power to adapt to the impedance of the headphones that I have, 4, 8 and 220
The intra delivered with the rog5 are those with the least powerful volume, the Sony is in the EU safety value, the others are above but do not exceed 100 dB (A), the maximum limit.
The ROG Cetra RGB are 16 ohm, it would be necessary to test these on other smartphones;
If the volume is loud on these, the Rog5 does not send the right power in 16 ohm
If the volume is low, the ROG Cetra RGB are designed not to exceed this volume, I imagine 80 dB (A), low limit of the standards but can be exceeded without exceeding 100 dB (A)