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.