At least with the RME ADI-2 DAC and Chord DAVE that are typically hooked up to my Phonitor, I tend not to use the Phonitor with IEMs. Both of those DACs have direct outputs (very direct in the case of the DAVE … it’s literally the same connection as goes to the RCA and XLR ports) and the Phonitor doesn’t seem to add anything there for IEMs.
For full-size headphones it’s a different matter.
That’s not to say I think the Phonitor sounds any “less good” with IEMs than either the RME or Chord sources, but it isn’t adding anything there for me either … at least with those two sources, and I’m generally a proponent of not having things in any given listening chain that aren’t necessary (i.e. audible discernible as being “better”).