[font=trebuchet ms]Search the GT archives, and you'll find many learned dissertations on the subject of power vs. loudness. I wrote some of 'em. :D

Doubling the power into the same speaker will produce a barely perceptible increase in the Sound Pressure Level. SPL is expressed in logarithmic units, in this instance an increase of 3 dB. If one speaker is more efficient than the other, this could contribute more than the power difference. If the 15 Watt amp has a speaker rated at 96 dB SPL at 1 Watt (the standard comparison), and the 30 Watt amp is driving a speaker rated at 92 dB SPL, then the 15 Watt amp will be slightly louder than the 30 Watt amp, until both amps are driven flat-out, and then they would become about equal.

More important would be any difference in sound quality. One amp may have more pleasing tone than the other. The loudness difference should not be an issue.[/font]