Its a logarythmic scale, so +3 db is much more than 3 times +1 db and would indeed be noticable. Some claim to be able to hear as little as 1db difference in levels.
I would be interested to hear Johnk and Alan's opinions, but I would think that if you are in a situation where you are close to the limits of your receiver, it would be possible to hear more subtle effects from the clipping of peaks in music with a lot of dynamics. In such a situation another 100w might not get you a lot of extra volume, but if it eliminates clipping of peaks, you would hear it.
