Just forget about the impedance with IEMs, its always very low. The sensitivity is the more important factor that determines how loud they play. A high impedance headphone can be easier to power than a low impedance pair if the sensitivity is high enough, your earphones are very sensitive so they require very little power to get loud, a pair with lower sensitivity requires more power, you regulate the power with the volume control which will need to be adjusted with each IEM to achieve the same level of loudness. A typical IEM has sensitivity about 105db/mw, an IEM with unusually low sensitivity like the Final E5000 has sensitivity of 93db/mw, the numbers may not seem like a big difference but it is substantial, one will play loud on anything, the other generally requires some external amplification.

The impedance of most IEMs makes very little difference to the loudness but, if you are getting a lot of unwanted noise with a very sensitive IEM you can use an impedance adapter which should tame it, I use an 80ohm adapter with one of mine, you need to really boost the impedance to get a substantial difference.