How do I keep my cats off my amplifier?

Build a cage from stiff wire that rests on the floor but covers the amp and stand.

Couple of alternatives the first is in the same thinking path of Onhwy61 make the surface less friendly: do you have any extra brass cones you're not using? Place them on top of the amp, try to leave little space for the critter to cuddle as a bonus you get mass loading too. Think Audiopoints ows my a beer for this one!
On the cheap get kids jacks and make a mine field for the cats choose any pattern you like , there you go hope your cats are not related to fakirs
Second one is to use a cat repellant there are some granules that can do the trick and also are "sonic" repellants you can also use....
How does this sound?

Couple loads of buckshot ought to do it.

arrrghhhhhh!!!!!!!! cats on amps, seems like we hear the cat ate my stereo deal quite a bit, f*#* cats.

get a big lazy dog & just throw the cats in the garbage while you still got a hifi.

mike.

Slap the cats with a sort of whip, everytime they get on the amp. Make sure that the whip does not have any metal pieces in the end...When you "whip" the thing in the air, I am sure the cats will simply stop in their tracks and get off. The best thing to do is to simply do this whip routine everytime and then do so sporadically, so the cats do not know when it is coming. It is better than the pellet gun, the dogs(unreliable for cat chasing) and the poison alternatives. Remember, we want to keep them felines off and NOT kill them off.