The system is fairly intricate and my article spends a great deal more on the questions you ask than can be writen here.
The amplifier is NOT adjustable. But, as I wrote there are an almost infinite combination of other possible adjustments. Read the article, understand how the system works and do the adjustment 10-30 times and school will begin. The article explains what the amplifier inputs are and their inter-relationships. It starts with the basic system which really never changes. You must understand that first.
Bosch Master, ASE Master, L1
33 years MB technician