Wattage has very, very little to do with how loud speakers actually get. The primary factor to determine how loud something is is actually the sensitivity (or efficiency of the speaker). A speaker rated at 90dB will produce music at 90 decibels from 1 meter away with 1 watt of power. Generally speaking, the human ear can register a change of +/-1dB. An increase of 10dB is what is required for the human ear to hear a doubling a volume.
And the lower the sensitivity of the speaker, the more power it takes from the amplifier to reach the same levels of volumes. For example a speaker that is rated at 85dB takes 2x as much power to play at same volume levels of a speaker rated at 88dB. So, this efficiency is extremely important to consider when determining volume levels.
If you take a stereo amplifier that produces 50 watts and you compare the maximum volume levels to an amplifier that produces 100 watts of power, you will not hear a 2x increase in volume. Instead, you will gain approx 3dB...which is noticeable...but not dramatic. The general rule of thumb is that you would need 7-10x as much power to generate a 10dB increase. Thus, if you want to ensure 2x the loudness level of a 50 watt amplifier, you would need a 350-500 watt amplifier to accomplish it.
