The basic premise is that by pushing the amp beyond the design capabilites, you risk damaging any down stream device. Typically these are speakers, but are not necessarily limited to these. However, speakers are in the original question, so lets stick to them.
The mention of lower power amps in this context is as mentioned previously. People get heavy handed with the volume control, and drive the amp past the logical point that it can supply quality output. As Milzie mentioned, this can also happen with "other" amps. Typically mass market companies develop and design equipment that is rated very " enthusiastically". But because they are mass market aimed at price points, many design (flaws is such a harsh term...) trade offs have been made.
Specifications are typically subjective, and some manufacturers test their equipment in a lab. Amp output can be measured by driving a "dummy load" which is not indicative of changing impedance of a speaker. A speaker is a complex array of resistance, inductance and capacitance, and the mix changes with the frequency it is asked to reproduce.
But hey, we are now treading on Ian's area of expertise.....oh wise one, comments please?
Regards,
BBIBH
