Note that 600 Ohm headphones typically need higher voltage for the same loudness. That's the only thing special or different.
...Impedance and resistance (both Ohms) is "the resistance to current flow". So with lower impedance you get more current. And power (wattage) is calculated as Voltage x Current, so more current (more voltage and/or less resistance) means more power. The loudness depends on how efficient the headphones are at converting electrical power to acoustical power. ...We don't normally measure acoustic power, and headphones normally have a sensitivity rating, not an actual electrical efficiency rating.