I'm planning to get a sony NW-A55, does it has enough power to drive e4000 ?
It's hard to say since measurements aren't linear. But with the numbers they give on paper, no. If you look at E4000's impedance and sensitivity they require 2mA to drive to 85dB. That's still louder than I would normally listen. The N55 outputs around 1.4mA. So at levels under 85dB you're probably fine. I used an A45 that has the same output power with E4000s and thought they sounded good. I didn't have anything to compare them to except Audio Technica AHD700s. But they still sounded good to me. The E5000s require 50% more power than the E4000s (2mA vs 3mA). But again, these are ballparks since it's not all linear.
http://www.digizoid.com/headphones-power.html