Hi Steve,
Welcome to Audiocircle and the RWA forum!
I can iMod any of the iPod Photo units. These are also called "color iPods," but they are NOT the Video iPods (5th gen). There are also the 4th Gen iPods with a monochrome screen (no color). They look just like the iPod Photo but the screen does not have color.
Yes, you feed the modded iPod's line output (constant voltage) into a headphone amp and that powers your headphones.
You cannot connect headphones directly into the iMod because the headphone output jack is converted into a line-out jack with a constant voltage (which is too loud for headphones AND not meant to drive the low impedance of headphones).
Hope this helps,
Vinnie