Just for Kicks I put 3 new AA batteries in my old AIRHEAD portable headphone amp. An early Airhead. Not a Total Airhead. 
The one that looks like this.
I connected my iPhone, and my HiFiMan HE-560s. And it drove them pretty well. Used Neutron player and listened to Chessky Audio Sampler a bit. Levels were adequate, really adequate. The little switch for crossfeed works - and with these phones I heard it like never before. For about the first time, I decided I didn’t need it.
Now I’ll put it away, but like I say, just for kicks. Sometimes the old stuff surprises you.