Not nearly as sensitive as my cIEMs.
Sensitivity is not the same as impedance, I mean the dB/mW or dB/MV, thus the sound pressure generated per an amount of energy.
The RE600 delivers 102 dB/mW, while my cIEMs deliver about ~120 dB/mW.
I actually sold the ibasso DX50 because I had quite a bit of noise when using it with the cosmic ears BA4f.
Something I now regret, because the RE600 should have little to no audible hiss with the DX50.
Output volume depends both in sensitivity and impedance when it's measured in dB SPL/power. When measured in dB SPL/voltage, it is only dependent on sensitivity.