Here are some frequency response graphs for the various models of iPhone.
(Note these are of a complete loop – microphone recording AND headphone jack playback – they include the frequency rolloff of the microphone hardware.)
Flat is good – so the original iPhone wins! Then we gradually decrease frequency response as newer iPhones are released.
Look at how bad the low frequency roll-off is on the iPhone 3Gs. It’s -3dB (half the output power) at 200Hz – no wonder it sounds so thin.
The iPhone 4 response is a slight improvement over the 3GS.
The iPhone 5 frequency response is similar to the 4S.
Notice it’s the original iPhone that wins. That was about the time Apple made the best iPods too.
This graph came from http://www.faberacoustical.com