I started to notice that with the C3 I can hear some subtle things in my better recordings at a lower volume level than the Image when coming out of my iPod. At first I thought it might because of the separate tweeter/sub design but how much could be the impedance different? I admit I don't have a firm grasp on impedance. My mini amp is out being updated to work with IEMs so I can't test with more power and see if that helps. 

I believe that the C3 impedance is 32 ohms.

 Donald1800

I think the C3 is 32 and the Image is 50. What I was wondering is could that account for why I tend to increase the volume with the iPod. Or is that too close for the average person to hear and the difference is the hardware. 

I don't think that the impedance is causing any effects.  32 ohms is considered a medium impedance, I believe.  The effects you describe most likely are caused by the inherent device sensitivity and accoustic frequency curve.

Donald1800