let me try it.
I'm running my E7 off the laptop as a DAC only, and running a line out from the headphone jack on the E7 to the input on my headphone amp. That being said, is there any benefit for me to use the L7 output than the E7 headphone output running into my headphone amp?
your current setting, E7 internal amp part still being used. in another words, the signal from E7 line out headphone jack is still amplified by E7 internal amp. then goes through your headphone amp to amplify second time. E7 amp is not great and could introduce distortion due to space limitation, so by pass E7 internal amp will give me much clean signal source to amp, and isolate E7 as pure DAC. DAC part in E7 is decent.
my final answer to your question is YES. of course. I assume your external headphone amp quality is better than E7 internal amp..