A DAC will improve the sound of any source, but you don't necessarily need one, and you don't need an amp to drive this cans, but as with the dac won't hurt having one. I use it with my xperia z3c, pc, and with a xduoo DAP without an amp.
The impedance is low, and the sensitivity is enough that you *should* reach fairly loud volume with only 25-30mw of power, but the weird thing is that a lot of people report low volume on phones (especially iPhone) and without an amp. While on paper the specs should be fine and easy to drive, I suspect that because of the low impedance many headphone outputs without an amp lack the current (amperage) capability these headphones want. As you go lower in ohms (for headphones) the amp needs to have more and more current capacity. This is often a problem in larger speakers and amps say things like "do not connect 2ohm loads to amplifier" or you will see some power amps that actually advertise they have lots of current for low ohm load driving capability. There are even a lot of headphone amps out there that if you talk to the manufacturer they say the minimum load is often between 15-25 ohms depending on amp model. So anyway, kind of rambling here but I suspect these gain so much from an amp because of their extra low impedance. That's not to say you MUST use an amp, there are many people here who do not. There are just too many variables to say if your particular setup will work perfectly fine without an amp or not. Just try it and see.