Ray Samuels products come to mind.
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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).
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.