I am just trying to be helpful here
@justDAP, so please take it that way. One cannot hope to compare how powerful two devices are by comparing the setting on their volume controls for a given SPL. There are simply too many variables.
The first one is how much drive the device is getting. In this case the input to the pre-amp from the DAC section.
Two, the taper on an analog pot or the selected step magnitude for a digital volume control will normally have differences. Sometimes large differences.
The only way to do such a comparison is by measuring the power output required to reach a given audio level. At least this will almost certainly show that there may be a difference of some amount of numbers registering for the volume control on each device, the power output is the same. Of course if we are using the same transducers in each case.
To know if one device is more powerful than another we would have to measure the maximum power output at some standard impedance when it crosses a certain threshold for THD, or some other parameter that is important to us.
A quick search shows that the X5III is rated at 250 mw into 32 ohms. The DX80 260mw into 32 ohms (though I did not see distortion levels for the DX80, the Fiio is pretty high at 1% for rated output.)
So the real available power difference is negligible.
It is a similar situation where we read someone saying that a device has lots of headroom because it only needs 3/4 volume to get loud enough. In this case, we have no idea whether the next click on that volume control is going to push the device into clipping.
Basically the setting on the volume control cannot be relied upon to tell us much of anything, other than an acceptable volume level with a specific song, on a specific device, with one specific set of IEMS or headphones.