Dynamic Headroom

A much larger power supply is required to add headroom to an amplifier and the power supply is where the money is.

Although as I indicated earlier, headroom can also be "added" by subtraction. Either by underspecifying the continuous power capability of the amplifier, or by designing it such that its continuous power capability is reduced relative to its short term power capability. Doing the latter would likely reduce the cost of the amplifier, rather than increasing it.

Regards,
-- Al

Real bench measurements will find these out pretty quickly. It can work against a manufacturer, too, if they rate an amp at 40x2 rms and it will put out say.....80x2 rms. As has been noted, this will also artificially inflate the dynamic range spec. Most people looking for an amp in the 80 watt range will skip this amp unless they manage to run across the real, measured power.
For example, all Bryston amps come with real bench test results. The advertised spec is guaranteed minimum. All Bryston exceed there minimum ratings.
The headroom under unspecified dynamic conditions is icing on the cake. I don't know how much Bryson amps exceed there RMS rating, but it probably is only a fraction of a db.