as you can see from the content, the Q701 can output 120dB sound pressure when the input power is 32 mW . so E17 should drive it quite well, remember most mobile phone can only output 6 mw power.
for some hard to drive headphone, we need to check the max output voltage and the current, you can see the specification from our website www.fiio.com.cn.
The output power of E17 is 220mW at 32 ohms, so the power should be about 110mW at 64ohms, still bigger than 32mW. and the max peak output voltage of E17 is 7.3Vp-p , max output current is 80mA.
So it should drive Q701 quite good. of course, it is just for reference, and this is just based on a person who have average hearing.
What does the peak voltage and max current mean in that chart?