As per TheSloth, lower impedances can cause a more rapid roll-off at low frequencies. He observes a noticeable roll off below 50 Hz at earphone impedances below 32 Ohms. Good ears! At the other end, lower impedance equates, for the most part, with lower resistance. Thus, the phones may sound louder, but may also exhibit hiss if the impedance drops too low. Simply adding a volume potentiometer (eg, from Radio Shack) should increase the impedance a bit and reduce the hiss a bit. The Ety ER4S is approx 100 Ohms, and was originally designed for home stereo systems, not portables. The ER4P, for portables, is approx 27 Ohms.
Unfortunately things are not so simple. Impedances are not static, but can vary across the frequency spectrum. Phase will generally vary inversely to impedance. Crossovers will also impact these parameters, usually negatively.
As per the iM716, I wish it were 16 Ohms, like the Ety ER6i. They’d be a bit more sensitive. I measured the DC impedance of the iM716 at approximately 68 and 37 Ohms at HD and Bass modes, respectively. These values were confirmed by kwkarth. The impression has been that the difference between the HD and Bass modes is simply moving a set of resisters in or out of line. The paradigm for this concept is based on the Ety ER4P (27 Ohms) to ER4S (100 Ohms) converter simply being the addition of a set of resistors in-line.