Damn, this sounds like an extremely solid choice for those getting into this hobby. I'm looking into a higher level replacement to my E7/E9, but I'm not against selling my E7/E9 (or gifting it to a friend) and using the E15 as a secondary unit to my inevitable purchase of the Schiit Lyr + higher end DAC.
I'm extremely loyal to the Fiio brand due to the quality products they put out at the price point they're placed in. I'm very much looking forward to the E15.
Suggestions:
Low/High gain switch in FRONT, like a slider or button.
RCA coaxial input (red/white) instead of 3.5mm. People are picky, and would prefer not needing an RCA to 3.5mm adapter. Dunno if you covered this already
Less illuminating led for as a power indicator, and placement considerably farther away from the volume pot.
Better 3.5mm headphone input. The E9's 3.5mm is extremely rough, and can be a chore to plug stuff into it.
Please make sure there is ZERO noise when adjusting the volume. Almost everyone I know has this issue with the E9. Not a dealbreaker, but a minor annoyance.
More transparency than the E7/E9. A lot of us would like to have an amp that is mostly like a 'wire with gain', where it doesn't impart it's own sonic signature to their headphones. I'm sure this was more the E7 adding a little bit of warmth, moreso than the E9 adding anything.