I agree with
One thing I would like to add is that if the X7 is the masterpiece of FiiO engineering, the ultimate statement product, that you shouldn't need to in effect have a daughterboard of an external amp...
Nothing wrong with the X5 / E12 stack, but - having the E12 (or - better!) internals (minus battery, as that could be shared) inside the X7 as a one-box solution would definitely win you some new fans...
I agree here - just got my new X5+E12 combination - very nice, but bulky as heck! I would also like to see a separate treble and bass adjustment (like on the E17) - which is sorely missing on the X5 - that's why I got the E12 (for the bass boost!)
Keep the dual MicroSD cards, OTG, USB DAC, etc.