In regard to the E10K and the recommended headphone impedance; some people take that literally. I guess that is understandable, but not everything in the audio world translates from what is on paper or a website into what is heard, if that makes any sense. The E10K can drive headphones with a higher impedance pretty well, such as Sennheiser HD600/650 for example, which are 300 Ohm. Heck, even the FiiO X1 can drive the 600/650 pretty well. I've got both an X1 and HD600's and it does a decent job. X1 has less power than the E10K.
Impedance rating is just a guide really. There are other factors to take into consideration when determining how easy or difficult any given headphone is to drive. You can't go on impedance alone. Some specs aren't given and the average person has no idea of what to look for, so most people just by the impedance rating, but some take it as a rule I guess.
As an example of impedance rating not telling the whole truth as to how much power the headphones in question might require: AKG K702, which are 62 Ohm require four times as much power as Sennheiser HD598, which are 50 Ohm to reach the same dB. K702's need twice as much power as the 250 Ohm DT770/990 to reach the same dB, despite having a quarter of the impedance.
I think there are some Hifiman headphones that really need a speaker amplifier to power them properly. That's quite rare though.