Portable Headphone Amp with Wall Charger

Ray Samuels products come to mind. 

I bought Oppo HA-2 headphone amp recently and it sounds amazing. It has async USB, SPIDIF, DSD an analog in/out. I use USB. Sounds better than my Benchmark DAC1. D/A converter IC is SABRE 32-bit. It has lithium battery (7-8 hours) and can be used with a phone, but I’m using it as stationary unit only (next to my computer).

https://www.oppodigital.com/headphone-amplifier-ha-2/

You can charge it with any micro USB charger and connect to iPod touch with provided short cable. It comes with rapid wall USB charger. I used it with my Samsung phone and it played beautifully. It should be able to drive your HD700 - compare power ratings at 300 ohm.

the Oppo looks very interesting.  How do I charge it if I do not have a computer? Thanks