You don't need to see the voltage because it probably would mean anything to you, all you need to know is that your amp and HU are matched up. Even the tools to set gains don't show you the numbers, you basically set them by the shape of the soundwaves, an o-scope shows you those waves so you set them visually according. And the dd-1 does the same thing but instead of showing you the actual wave on a screen, a light just turns on at the clipping point so you know.
And yes iPods and mp3's can be clipping, anything can be! Even a song itself can have clipped bass. If you listen to an iPod all the time in your system set the gains according to your iPod, simple as that. As far as music goes try to always download the best quality of music possible. An if you are getting bass boosted stuff make sure it's from a known composer that doesn't have dirty clipped shit