TalbotYou Always need a headphone amp to get the most out of your cans. If you're plugging into a PC with a decent sound card, maybe not, but smartphones and most but high-end DAPs do not have an output amplifier that is really capable of driving headphones and doing so well.
At least get a FiiO E6 and see the difference even the cheapest little amp (that's actually built to drive headphones) can make.