Apparently the ER 4Ss have moderately high impedance, is this marketing rubbish then? And the headphone amp in the gear is cack so I'd rather not use it.

The impedance is the difference between the P and the S. The S sounds better with an amp, but needs more power to drive it. But ety (and a number of other people) sell 75ohm adapters to convert ER4Ps into ER4Ss, which is why I recommend the P instead of the S.

Impedence is only one factor. Sensitivity is what matters as it's the combination of voltage, current, and impedence that drives the measured dB for sensitivity. Take that, double it for margin and measurement BS, and you'll find that it's really not much power necessary at all. According to Ety, 108 dB SPL for a 0.79 V input, (90 dB @ 0.1 V; 100 dB @ 1mW). Any opamp with a resonable voltage supply will be more than enough for IEMs. Problems may arise from not enough voltage (1 AA/AAA battery for instance) and circuitry in the way, as well as the opamps used for the 1.2V swing in some portables.

A CMoy has none of these restrictions and with a 9V NiMH battery and the AD8599, you'll look at gobs of hours of runtime for the field (over 60 hours easy).

For the ER4P it is the following 108 dB SPL for a 0.2 V input (102 dB @ 0.1 V; 106 dB @ 1mW)

But, but, don't you NEEED the RSA Tomahawk for IEMs?

After all the IEM-fairy has told little Ray a special secret to make his little wonder something special for IEMs.....