I have a rather simple question, what amp should I use with my Nexus 7? I would like to stay in the $150 price range.
There are lots of "portable" headphones that can be easily driven by the Nexas 7.
If yuo still want a amplifier, the Fiio E11 portable headphone amplifier ($60)
10 hours (best guess), may depend on what voltage and gain settings you use with the E11.
For a few extra dollars, you can get a spare battery (BL-5B) or two and a simple AC battery recharger.