Bottom line is it's really a matter of what you like and how your ear perceives the sound. Try Sennheiser IE8, those are really nice, worst case you can always sell them used and still get your money back.

  Quote:   According to the manual, when AK100 was first released and the battery was fresh in the box, the device was rated for "about 16 hours of continues" lossless play (normal usage with clicking, scrolling and some screen back-light). For mp3s it should be a lot more but that also...

  yes, it looks like it came out of the same factory. definitely solved short playback time issue.

  I was only able to find it on aliexpress for $7.86 http://www.aliexpress.com/snapshot/6745429240.html (seller name: weiyu li). It is exactly the same battery, size and markings, as in AK100.    Its very easy to change the battery by your self, iRiver made it very simple. 

  Part number on the original AK100 battery is PR-665354, so the exact size of the battery is 6.6mm X 53mm X 54mm.   All markings are:   PR-665354 F12095201AA 7.4Wh 472244679069 3.7V 2000mah