I'm not sure about portable DAC's but I use my P5's with lossless encoded music from my iphone's line out, in to an O2 headphone amp - sounds just wonderful. The limiting factor will now will be the DAC circuit inside the iphone.
With the ear cushions I think a large factor is how much they compress against your ears since this will change the volume space inside them. Personally I have to extend the arms to almost Max when I wear mine and I find the force against my ears is probably a little more than it needs to be. Perhaps those with smaller heads don't compress the ear cushions as much, keeping the sound clearer?