Back in the Ye Olde Dayz, there was this thing called "dynamic headroom". It was the ability to squeeze the last bit of current from the PSU filter caps and apply it to the output transistors. How much more you could get for a few milliseconds (before you ran out of either current or output stage gain) varied from around 1.0dB or so to as little as zero.
These days, there is almost no such thing due to value engineering.
Not to confuse the OP even more, back in the '70s there was consideration (by the IHF IIRC) of a dynamic headroom spec, mostly in the context of reproducing wide dynamic range classical music (Like 1812 overture with cannon or mortar explosions). This was before the trick amp topologies like Class G/H were popular so peak vs continuos sine wave power was mostly a matter of transformer/power supply "regulation" ( regulation means how much the voltage sags under continuous current vs. no load voltage due to winding resistance) and power supply reservoir cap sizing that affects ripple voltage and clipping under power. The FTC even got involved with voltage sag after the transformer gets hot (temperature coefficient of copper wire increases resistance with increased temp causing more internal losses) adding a thermal preconditioning step before measuring rated output power.
These metrics like dynamic headroom and overly dynamic recordings did not gain much currency with consumers so faded away. FWIW modern amps with PFC and regulated supplies will not suffer regulation or temperature sag while a smart amp may reduce voltage in response to difficult local conditions.
Back in the late '80s I designed a small bedroom studio monitor amp for Peavey (AMR) that was a nominal 35W amp, but was capable of 2x the voltage output transiently, so 4x the peak output power... In practice that 4x power deteriorated quickly, so i specified it as something like 100W for X mSec, 60W for 15 seconds (limited on purpose to not overheat the amp), then 35W for 24x7. This was a kick-ass little amp, but the technology did not scale up to higher power points cost effectively, so that was the first and last model in that series.
This is an interesting (perhaps) diversion, but loudspeaker design and mating with appropriate amplification involves multiple variables beyond this simple discussion.