Default Is a Fiio E18 worth it?

Hey guys, I'm currently eyeing a Fiio e18 for my Nexus 5 (running CM of course )

Currently I run a pair of ATH-M50's but I'm planning on upgrading to a pair of HE400's within the next year.

My question here is if it's worth it to buy an e18?

I know both the m50's and the HE400's aren't exactly super power hungry so I feel like the e18 might not be required to use them.

Will there be a huge difference from the built in DAC to the e18, or is it just marginal improvement on the M50's and the HE400's?