Pono was the best SS headphone amp I ever heard

Mind you, I haven't heard everything, but as a headphone amp and DAC the Ayre designed output stage was phenomenal.

The only other headphone amp I liked as much was a $6K tube set up I can't remember the name of.

So my point is, if you like your music on the go, and you got to hear the Pono, what do you think sounds as good if not better?

I don’t like my music on the go.  I don’t like having something on or in my ears, the hole in the head effect, or since I live in Chicago, the lack of awareness of my external environment which could lead to being criminally assaulted.  I have 3 Stereo systems in my home and would avoid using headphones in preference to listening to one of them.  Nonetheless, it’s unavoidable sometimes.  As I write this I have a house full of visitors and am listening to my B&W Bluetooth phones before I leave for work.  When I travel I can’t bring any of my systems with me.  I have had a few high end portables, dabbled with Amps, etc.
  I did audition the Pono at a store.  It was the first high end portable I heard and I had brought my own Oppo PM3 phones.  It was impressive with and without an amp.  I was put off by the reports of difficulties in file transfer And subsequently bought a high end Sony portable the size of a cigarette lighter.  The Sony also sounds amazing for such a small gizmo.  A lot of time elapsed between the two 
listens so I wouldn’t dare to compare.  The Sony is virtually impossible to extract the little memory cards and I have reverted back to my I phone with Bluetooth.
Good thing you passed on the Pono. Support is almost nonexistant. What a fiasco.

I went with a Fiio. Much better UI, size and expandability.

But damn, the Pono's handling of headphones was something else.

Try the Ayre Codex but only with the balanced output.  Similar circuit that Ayre designed for the Codex.  Not very interesting until you acquire a special balanced cable to use with two 3.5mm plugs.  Same balanced setup you can do on a Pono.  Huge difference in sound quality. 
The Stax SRM-T1 or SRM-007T tube headphone amps, for their Lambda Pro Earspeakers. 
Sadly neither the Codex nor Stax are good candidates for "music on the go"