The above mentioned advice seems logic and true. I have mounted amplifiers on Sub boxes before and have experienced no loss or damage. Most of my installations were on systems of less than 300 watts. At higher power I can see how things can quickly change.
If you would like to mount the amp on the box because it would eliminate grounding/short circuit errors between wiring and/or you just need the room in your trunk, then consider using rubber busshings between your amp and the box to reduce vibration.