I know it's been quite some time since you asked this question, I just joined and wanted to get a little info from you. What is the density of your 20mm minus? Is it high in clay content? How many tons per hour are you wanting to screen of the 20mm minus? As funwithfuel mentioned, amplitude is often preset from the factory as well as the frequency, depending on your response there could also be other solutions for your application. I know I'm late to the party, if your still dealing with these issues please reach out.