How does the shape of the equal loudness curve vary with power level?

Answer 1

The ear is insensitive to low frequencies at low volumes. As the sound gets louder, the equal loudness curves flatten out until at 110-120dB or so the variation in sensitivity with frequency is greatly reduced.

In order to see what the curves look like, Google for:
Fletcher Munson (applies to headphones)
Robinson Dadson (applies to loudspeakers in a room)

Answer 2

Equal Loudness Curve varies directly with power levels!