If you want to protect the finish, Heli-tape is what you want. This is a colorless polymer film that was developed to protect helicopter blades, but it is very common to use it on mountain bike frames to protect from flying rocks and debris. This is likely stronger than the vinyl wraps used for cars.
To stop scratching, apply double-sided tape to the areas they want to scratch. Cats do not like that sticky surface on their paws and will move to something else.
The aluminum foil idea can work, they don't like the noise when they jump on it.
But, a better alternative is to get one of those office chair carpet mats that has all the little plastic spikes on the bottom, cut it to the size of the top of the speaker, and lay it up there with all those spikes pointing up. A cat will only jump up there once or twice, then realize all those little spikes mean that this is not a place for them to roost. After you get them trained that way, you can likely leave it there for maybe a month, then remove it, and problem solved.
The ideas for redirecting and giving more appealing areas are also good.
Just to be clear, I LOVE cats (and dogs and most pets). I have had my fair share of dealing with these little a$$holes and the items outlined above absolutely work!
