Originally Posted by cmbdiesel

More watts does mean more volume..... and more important it means cleaner waveform at comparable volume.
IMHO, 15 watts is completely anemic, and would not even be acceptable as a headphone amplifier, let alone into a pair of what is most likely 8 ohm boxes...
Cranking that little 15w rig up to a comfortable listening level outside, especially if someone is 'out in the garden' would likely result in severe distortion as you exceed the halfway point of the amps output. Very few consumer grade amplifiers put out a decent waveform above half gain, they tend to get really bad above three quarters... at max, it's basically square. Then you're replacing speakers...
Personally, I consider an amp that is rated for twice what the speaker can handle to be barely adequate.

YMMV