First, thank you to ClieOS for an excellent review!
I managed to get a hold of a FiiO E7 and am really liking this amp. I have been mostly using a iBasso T2 but find the E7 much better.
I am using it with Hearyourself custom C3 IEM's and Audio-Technica ATH-M50. This amp has no problem between the two.
ClieOS has it right in describing the bass boost as adding warmth to the sound. With my C3 IEM's I usually switch between LV1 and LV2. With the M50's I usually switch between LV0 and LV1. This is determined by what I am listening to.
As a DAC, well it does a great job! I have used it with a MacBook Pro and have been pleased with the sound output.
When this becomes available, I would not hesitate in recommending you give it a try.
My only issue is the size, but I am accustomed to using the ibasso T2 which is very small, and changing to the E7 will take some getting used to.
I will learn to live with the size!