How do I keep my cats off my amplifier?

Cats hate to walk or lie on sticky things, so put some wide tape on top of the amp, sticky side up (you can wrap in it a loop around the amp, so the sticky side never touches the amp). Use just enough so that the cat can't sit comfortably on the amp. After a while, the cats will probably come to perceive the amp as an unpleasant place, and you'll be able to remove the tape.

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.