I had the same complaint regarding output volume on these icon mobile using full size headphones. Though they do sound good, it didn't produce a higher volume than my iphone 3g so i email the folks at nuforce and got this reply:
"Hi Paul, Yes, you are correct that the maximum volume with iPhone is not louder. The Icon Mobile's gain is set to 1x gain, and at the maximum it cannot become louder than iPhone, the Gain switch at Lo setting would bring down the volume. If your headphone is very inefficient, they may not be enough boost, you're free to return the IM to Nuforce. Regards, Casey Nuforce"
For this reason i returned the IC for a full refund, thanks to jason for the help. I am now currently using the E5 and it does give a noticeable volume increase from the Iphone. Does anyone know of a portable amp that would increase sound output for use on full size headphones but not be soo bulky. I like the size of the IC and sound quality, it just didn't provide a higher output. T4 maybe?