Im not sure if my mind (or my amp) is playing tricks on me but...
I have been listening with a resistor adapter 240 Ohm on the MH1C and using an amp to drive it. I notice the bass quantity have tighten and less mid bass as well. Some might love this kind of sound.
I also notice a darker background and overall quicker decay of the notes which makes it perceived faster and more laid back. For this however, I suspect its also symptom of underamping (?). I'm using iBasso D4 Mamba.
Without the resistor adapter, I could get a good volume at 10-11 o'clock. WITH the resistor adapter, I get the same volume at around 2 o'clock (with less bass and mid bass) which makes me think that the amp is fine. I dont if its really due to underamping but maybe could someone help me explain whether my observations could be true?
Generally I have been testing the resistor adapters on my IEMs and I think they do affect the sound in some way, especially the lows. The thing is, im a noob and I have no science or any form of measurement to prove it.