They usually are too lazy, so they say "I clamped at 1245 watts on a 1500 watt amp wired at .5 ohms, this amp is junk..."
If they new about box rise, and realized that their box sucked, then they'd redo the box.
If you do it yourself, take the extra 2 minutes to calculate and factor in box rise so you know what you're actually working with.
Gimme a minute to grab a link for you, better write-up.