friend has a question, best portable amp and best way to connect it to an iphone 5, (lightning)
Not easy... and its hit or miss. For starters you need the apple CCK kit. But it does not always work with the iPhone-lightning paired with most/many USB DAC/amps. Some amps need to be run through a powered USB hub, but even then thats hit or miss just as well, and of course won't work portably. This is how apple is able to play god with the aftermarket product manufacturers for the iPhone/Lightning connected products.
As an apple macbook and iPhone user this is one aspect that irks me just a little. I like the Lightning connector physically, its robust solid feeling and it has universal polarity, but I don't like how Apple uses it to control the aftermarket product industry.
Fortunately (at least the 5C) has a decently good sounding headphone output, I find it perfectly well suited as a line-level feeding an amp. So in otherwords, just amp the headphone output. Adding an amp makes a noticeable difference with my HD650 which sounds muddy and severely rolled off above ~6500Hz straight out of the iPhone 5C, although it gets plenty loud.
[EDIT] Theres a long running thread on this in the portable source forum. Although a good percentage of the success noted there is with the iPhone 4 and its 30-pin CCK... not the lightning CCK. And just last week Apple updated iOS to 7.1... so that I think remains to be proven out.
Edited by kramer5150 - 3/16/14 at 11:18am
I am not 100% sure... IIRC there are some success stories in that other thread about Lightning => 30 PIN combinations. But even then I think that is hit or miss... depending on how the USB DAC/amp communicates with the apple USB/Lightning host.