Then for the 5 ohm case, if measured at the HD558 terminals, the load will require more power out of the amp and perhaps explain the higher distortion numbers... More than back EMF, the amp would have to work harder and provide more power as some of it will be burned in its output impedance.
It will have to output more voltage, but the same amount of current. This should not necessarily have a major adverse effect on the distortion for a typical solid state amp with strong negative feedback, unless the increased voltage approaches the clipping level. Outputting the same current and higher voltage into a higher impedance load can even reduce distortion, because the distortion voltage resulting from the same current swing (assuming that this is the main source of distortion when driving a low impedance load) is now divided by a higher signal voltage.
You can check the InnerFidelity amp measurements for some examples, where the distortion with a 32 Ω load is often less than it is with a 16 Ω load at less than half as high output voltage. In other words, the amp can output more voltage and even current, and still have less distortion because of the higher load impedance. This obviously depends on the design of the amp and the actual levels and load impedances (how much of the distortion signal is due to the voltage/current swing), so it is not always the case, but it seems to be fairly common.
