I've been curious about this for while... What are the highest impedance IEMs on the market at the moment?   There is plenty of voltage at the source to burn so sensitivity is not an issue, just want them to be high impedance so they can be easy on the output stage.   So far I could...

Bought a pair of M6 a couple years back for daily throw-around use, never liked the sound.   Granted I was coming from higher priced headphones (DT880, ER4P), I found the uncontrolled lower mid-bass resonance really affecting clarity.  I played with a good dozen of modifications from scotch...