ClearsoundThere's a standard 3.5mm headphone jack on the bottom and top. Bottom is the line in from your iPhone headphone jack. The device comes with a male to male 3.5mm audio cable for this purpose.
The 3.5mm jack on the top would then hook into your headphones.
The only thing about using it this way is you don't get the DAC functionality, just the amp (someone correct me if I'm wrong).