Don't know about IEM.
This is probably more than you'll want to know, but much of Dr. Toole's research on speakers at Harman is available to the public. Though some of it is behind an AES paywall. Dr. Toole has written several articles though for different publications, including Audioholics. And his white paper on
The Measurement and Calibration of Sound Reproducing Systems, which details some of the studies you asked about, is available for free download here...
For decades, it has been widely accepted that a steady-state amplitude response measured with an omnidirectional microphone at the listening location in a room is an important indicator of how an audio system will sound. This paper examines both small and large venues, home theaters to cinemas...
www.aes.org
I haven't read it myself, but to take an even deeper dive on this subject, I'd also recommend Dr. Toole's book on
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms.
It's been awhile since I read the above white paper, and some of Toole/Harman's other works, but I think some of the studies simply involved EQ-ing or calibrating some speakers to a spectrally
flat response at the listening position in a typical room. And then giving the test subjects the ability to adjust the bass and treble where they wanted it for a variety of different music tracks, using shelving controls like this...