Some headphones do, but the LCD-2 do not. If a headphone has flat frequency response it means its sensitivity is the same at all frequencies. But its efficiency might not be the same at all frequencies - it might not have a flat impedance curve. However, orthos (including the LCD-2) do have a flat impedance curve, so their efficiency curve has the same shape as their sensitivity curve.
Consider the LCD-2: it has flat impedance vs. frequency and also has flat sensitivity vs. frequency. Thus it should require the same power to produce any frequency at the same level.