Lower impedance headphones are usually easier to drive.
Overall "loudness" depends on the sensitivity rating, as well as impedance. They are both key equal factors in determining how "easy to drive" an IEM is.
For example:
The Westone UM3X has an impedance of ~56ohms, and it's ability to be driven is similar to the ~30ohm Westone 3.
The sensitivity of the Alpha (101db at ~21ohms) is not too far from the Westone 3 - (107db).
Point is, these IEMs are all roughly similar in their ability to be driven (very easily) at varying impedances. Only problem is, the Alpha is so low on impedance, that it will sound very wonky unless it's run from a very low-impedance source.
The iPhone 5s, for example, has an output impedance of 2.3ohms - so using it with the load of a 20ohm Alpha might only *just* be ideal, and you may get variances in frequency response.
The 4-8ohm OI of the i9505 Galaxy S4, however, might be in trouble in this case.
Anyway, enough of my ramblings, I'm going to pick up one of these in the next few days, and put my money where my ears are - if it sounds good enough for workouts, I'll be stoked. I'm a Westone fanatic, they make great products.