depending on what kind of amp, mounting it on the box will totally mess it up.
Such as a large class d Korean amp, that uses spring clips to hold the mosfets to the heatsink. The amps are so big, with a lot of parts on the pcb's, the pcb's get very flimsy and you do risk breaking of feet from the mosfets.
So things like the big spl dynamics, american bass, digital designs, cactus, adassa, atomic, just for examples.