[redrich2000]

Just got some Hifiman RE-600 phones. They have a balanced mini plug. The balanced plug doesn't work in my iphone or ipod, but it does work in my Headstage Arrow amp. The Arrow doesn't have balanced output AFAIK, so I'm wondering what's going on here?

 

[Mad Max]

The pinout is incompatible with those iDevices, but the Arrow's simpler, regular headphone jack has no issue.  The negative contacts have to both short to ground for the RE-600 to work in a single-ended device.  They are the first two rings starting from the body of the 600's 3.5mm TRRS plug. iDevices and smartphones use TRRS as well, but the first two rings are used for ground + microphone, not L- + R- like a balanced device.  Your Arrow just connects those negative contacts to ground, so it works like a regular earphone.

 

[BRCMRGN]

You can buy a TRRS female to TRS male adapter for use with a smartphone and some of the Digital Audio Players that allow for a button for track control (like AK100). 
 
http://www.amazon.com/YCS-Smartphone-Headset-Microphone-Headphone/dp/B00MV2PDCA