Actually, I think 2 ohms should be fine for most anything Why would anyone design an IEM with an impedance that drops much below 16 ohms? So almost every DAP can become non linear? The old TF10 drops to 8 in the highs and it's an issue and a reason why the diversity in opinion on them. No DAP could drive them properly when they appeared. How is that reasonable? What were sony thinking with the XBA-4. A ground up design that can't be linear with their own DAPs.
The accepted spec for DAPs is to drive a load from a low of 16 ohms to whatever they choose for a high side. That means an output impedance of 2 ohms or less is optimum. Not designing phones to meet that is ridiculous with the number that get used for phones etc with much worse output stages. All that said, there's still some wiggle room as a a db or so variation may be less important than other aspects of a design but why go there?