[lutusp]

> The issue here is that Dell sound cards output power (that can be approached by a factor of the quadratic of the amplitude)

This is correct, and for those who want to know the technical reason, we can look at the Dell speakers (any speakers) as a resistive load (only approximately true). For a resistive load, the power increases as the square of the applied voltage, because the power in a resistor is equal to the voltage times the current, and both the voltage and current are increasing as the audio output level is increased.

In terms of Ohm's Law: P = E^2/R.