I'd think for going on the cheap if you can, however you have it fall apart, some kind of powder could be used, baby powder, flour, that kinda thing, that when it falls apart "explodes" a poof of dust goes out. Another speaker mounted behind it could give a really good sound effect of it falling apart. If someone is standing close to the fake amp they could possibly put a foot on top of it when it goes, releasing a catch and causing the whole effect.
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