I can see a lot of misunderstanding on what AD8692 is doing in E17, which is not there to drive anything. AD8397 is the opamp that drive the headphone. Only the last opamp 'see' the load and able to drive it. You can chain up 10 amps together, but at the end - only the last amp is driving the headphone and the maximum power the headphone will received is determined by that last amp, not the 9 other amps before it.
Also, sometime extra opamps are used after the DAC as I/V or LPF stage. Those opamps don't drive anything as well. Sometime it can be a Gain + Buffer design where the Gain stage handle only amplification and the buffer stage does the driving. Other time, it can be a DAC buffer followed by an amp stage, where the buffer stage is there to protect the DAC. It can also be a differential-to-single ended stage before than amp stage. Basically, it is not always as simple as 1 + 1 = 2.
Edited by ClieOS - 6/4/13 at 8:52pm