Is a Fiio E18 worth it?

I'd say try ViPER4Android and you may not have to spend extra money for a DAC

Viper4Android is a nice software solution to provide some advanced EQ features, but it's not going to compare to a USB DAC at all.

As far as power goes, the E18 isn't necessary. The HE400s have a similar impedance to the ATH-M50s, so if you're happy with the volume now you'll be happy with it with the HE400s. You may want to consider it for the quality though, since the DAC on the Nexus 5 is not the best. That said, while the E18 is great the E07k is about 60% of the price and would be a very good option to pair with those headphones.

One other thing to keep in mind, the HE400s are open backed while the ATH-M50s are closed. The HE400s are great if you are listening in privacy but you will get very little isolation and everyone around you will hear what you're playing.

I have the fiio e18 dac and it's money well spent. You need good files and headphones. Viper can't make up for a crap dac. Sure if you listen to standard mp3s with $40 buds Viper sounds good. Buy it from Amazon and send it back if you are not blown away, as I was

From my mobile telephony device.

If you aren't sure whether you want the E18, why not buy the HE-400s first and see how they sound straight from your Nexus? You can always pick up an E07K or E18 later on. If you have a local shop that carries Fiio, they'll probably let you bring in your Nexus and headphones and do some A/B testing to see if an amp/DAC is worth it.

I'm using an E18 and HE-400s with my old S3, and I'm very very happy with the combo. I haven't tried it with my Nexus 5 though.