You can't compare rigs unless you take out some of the variables.

Speaker efficiency:
Unless you use the same speaker to compare amps, you can't be sure of the results. If a low-power amp has an efficient speaker, it can be louder than a more powerful amp driving an inefficient speaker.

Non-Linear Hearing Sensitiviy:
The perception of differences in loudness is logarhythmic. To sound twice as loud takes [u]10 times[/u] as much power. In other words, a 50 Watt amp at full power is only twice as loud as it would be when it puts out 5 Watts! And that doubling of loudness is the smallest change that most people can notice!

Psychoacoustics:
There's more to hearing things than just getting the sound to your ears. The brain does a lot of very complex processing on the signals from the ears. One reported effect is that the type of distortion produced by a tube amplifier is interpreted as being louder than transistor distortion at the same SPL!

Do some searching through the thread archives. Use the search icon at the top of the page. This stuff has been discussed here a lot. And there's a wealth of info on Amptone. Check it out.