X5-3 is smooth by its nature, it is a matter of preferrence, X5 the first was neutral.
Really hard to say what differences you'll feel if you're using E11K as an amp though.
What type of sound do you like in general?
That's an interesting observation: will you hear a difference if you listen via the E11K?
I wonder, as I used to own an E09K, it was merely a good amp, not a great amp.
To the guy who posed the question, does an X5iii sound substantially better than an X5iii? I suppose you'll get a better answer in the X5iii forum. I wouldn't mind knowing the answer myself as I have an X5i and have a bit of the upgrade bug!