Hi there, my two cents, with personal real experience.
Some time ago I was told this works something like this:
Less amperage than needed means the power supply may be giving power below the hardware requirements thus causing malfunctions in the circuit, most probably nothing will burn( maybe too sensitive components can burn in this conditions?), but your hardware will not work as it is supposed to, go too low and it may not even turn on :).
Higher amperage rating means only that the power supply can feed more components/circuits/devices without trouble, but usually the power supplies are designed to support just a little bit more than needed because of cost of parts and to save you some $ in electricity bills.
So, going with bigger amperage should be safe while you keep the voltage at the required specifications. I have done this without any problems in a couple of synths and other stuff as I can't get psu with exact values so easily in my town.
About voltage, if you go low, your device may not even turn on or if it does, it probably will behave erratically. Actually some manufacturers take advantage of this in their devices(some pedal effects and tube amps I think).
BUT... if your feed your device with more voltage than needed, you risk your device getting damaged as the extra voltage needs to go somewhere
, usually as heat, and extra heat will damage components in time. I Also have done this, running into problems of course hehee, had to replace my mb6582 lcd after burning it by feeding 5v instead of the 3v it needed. So... don't ever go with a psu with higher voltage than required!