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.
Here is an article which goes into far more depth;
Stereo Amplifier Power - Amplifier and Speaker Efficency - Amplifier Power and Speaker Loudness
And here is a thread that discusses this as well as it relates to guitars;
One other thing worth noting, is there isn't a standard for measuring the power output of receivers that is reliable. Some manufacturers like Sony usually max out 1 channel and then just assume 110 watts (max from 1 channel x 7 channels) = 770 watts. However, the problem is the amp does not have the current to even come close to supplying continuous power to push all 7 of those channels anywhere near that max. It's likely to max out around 7-10 watts per channel before it clips so bad it blows speakers.
On the other hand, some manufacturers (like Harmon Kardon), actually list their power like this 65 watts per channel with all channels driven. See this link (http://www.harmankardon.com/EN-US/Pr...PID=AVR%202600
). But would you believe some people won't buy a Harman Kardon because it's only 50 or 65 watts per channel and instead they just get something else listed at 100-150 watts per channel. Oh well, that's marketing for you.