Running some quick numbers here making a few assumptions -
10 hours average run time
1300mAh battery
= 130mA average current draw
1v rms constant output
32ohm (headphones) and 10k ohm loads (amplifier)
The 32 ohm load uses ~25% of the total battery power
And the 10k ohm load uses 0.8%
These are real quick and dirty numbers here, only really points out the difference in power used between headphone and amps and also it is probably grossly excadurate the 1v output as that would be very loud for headphones, I only picked it as it was easier to work out and reducing that by 4, to 0.25V shifts that value down to only 6% of the battery power used by the headphones, but it is still quite clear that 6% is significantly higher than 0.8%
And for amplifiers, the difference between having the ipod turned down instead of full volume basically wont save you any battery at all that you could ever notice, and the loss of dynamic range is probably harming the sound quality in the process, so run the source at full volume into your amps and do the control there
