I use an E12 to tame the Sony XBA-4, which dips down below 10 ohms impedance at some frequencies. The low output impedance and low distortion is what I need as the extra wattage is overkill. The E12 definitely makes a difference due to the complex impedance curve of the XBA-4.
I have used the E12 with other IEMS (VSonic GR07 BE is the other high-fidelity one I own, but I've tried lower-end models too), and I've never had problems with hiss at the E12's low gain setting. I don't think you'd ever need the high gain setting of the E12 unless you are driving Sennheisers with 300+ ohms impedance or inefficient planars like the T50RP.