Here’s a good test to determine where you need amplifier isolation or not:
1. With no music playing, put your ear near one of the speakers.
2. Now reach over and whack the face of the amplifier with a rubber mallet.
Did you hear anything coming out of the speaker? No? Good, that vibration you just introduced is many, many orders of magnitude greater than anything that will normally occur in your listening environment. Therefore, you don’t need isolators.
If you're convinced you need to do it then try squash balls of different 'bounciness' levels. you can tell the difference by looking at the colour dot on each ball. The idea is that beginners use bouncier balls so they have a better chance of getting a game going and pro use less bouncy as they obviously have greater skill levels.