Just to be sure, what i meant is using LOD cable, plug 1 end into E5 heaphone out and other end into E5 line-in or using mic plug into E5 line-in to measure headphone plug into E5 headphone out. My E5 Frequency response roll off at 200hz on both ARTA and REW ( i got full range using E5 headphone out and Laptop line-in with both LOD and mic).
Sorry, English isnt my first language.
What you are referring is usually called a 'loop back' test.
The reason that E5 doesn't seem to perform well in a loop back test is probably due to its internal design. As far as I can tell, trying to use E5 to record it own output always result in very significant distortion, even over a headphone to an actual mic. The ADC and DAC seems to interfere with each other during a loop back.