I've done alot of Googling and looking at threads but every time I try to do the calculation the answer doesn't make sense.   My headphones have 30 Ohms resistance and have a sensitivity of 100 dB @ 1mW. Source is an iPhone 6 at 63%. Don't really know the maximum output because it is different...

So I've had my K545s for about a year now, and I'm very happy with them. I don't have any DACs/Amps and drive them out of my iPhone/Macbook Air and out of an Astro Mixamp when I'm gaming. My main gripe with the K545 is that wearing them for 4+ hours gets uncomfortable and hurts my ears...