not worth doing that syk0tic, MKB is right
lets say your amp is 2000 watts
at 12 volt, you need ~166 amps to pull that, lol good luck finding a computer power supply that going to pull that many amps. if you look at the specifications on your power supply itll probably say something like 20amps for a 500watt power supply im just guessing, and if you were able to find some thousand dollar power supply that could do it youd just rattle your house apart.
you were right to think hooking it up to a power supply would catch it on fire, its not rated for that and would either fry or catch on fire. the wires that would power the amp would be too small. plus the power supply isnt going to do shit if its not hooked into a computer.. so youll have to mess around wtih it and make it think its plugged in and you might catch it on fire that way, lol.