Hi gerogelai57, Yes, but you've got me wondering if you aren't plugging your IEMs into the D100's Line Out instead of into the Headphone Out. I apologize for even suggesting that possibility, but want to rule it out, as you haven't actually said that you're able to use the D100's volume control, at least to some degree, to attenuate the power, so here goes...
Does the volume at your IEMs change at all when you use the D100's volume control?
The output of just about any DAC's Line Out can be much "louder" than the headphone out, with typical desktop DACs having outputs of 2.0V rms, and portables like my PCM-M10 having a Line Out output of 1.0V rms. I don't know what voltage you get from the Line Out of the D100, but most likely it's 1.0V rms or higher. This can be really loud for most headphones, never mind efficient IEMs.
But whether you've accidentally used the D100's Line Out or not, most headphone amps actually play a role of reducing the voltage that's coming in from the DAC, rather than amplifying it, while also adding current (milliamps) to deliver the final power output (milliwatts) to your headphones.
(Inside my recliner's side table.)
Headphone amplifiers don't necessarily amplify anything.
My Metrum Acoustics Aurix amp (on top in the photo) has a toggle switch on the back that allows me to select 0 dB of gain or 10 dB of gain.
At the 0 dB setting, I'm listening to the DAC directly, with only the volume control and some impedance matching inserted into the signal path - with 100% of the actual power coming from the 2.0 V rms provided by the DAC (a Metrum Acoustics Octave MkII - in the middle of the photo)
and no power whatsoever provided by the Aurix amp itself.
I use the 0 dB Gain setting routinely to listen to my HD800, but I need the Aurix' 10 dB gain setting for my LCD-2 (which is actually much happier still with the power provided by the OPPO HA-2, balanced out.)
So yes, a headphone amp can be used as nothing more than an attenuator and impedance match.
The Low Gain setting of the OPPO HA-2 was purposely tuned to work with IEMs, giving you some finesse when adjusting the volume control.
Mike