The gain of U1 can be controlled by a digital binary 1248 nibble at ABC. The gain at digital 000 is unity or 1 and the gain at various stages are set by 4051.
There are eight different gains as the steps of gain resistor network is chosen by 4051. The on resistance of 4051 channel around 100E gets added to U1 pin 2 internal impedance.
You can use separate resistor networks with trimpots for each channel if you require but keep the networks total burden on U1 pin 6 to around 10K, not less than than. You can use this to set the gain of a amplifier with the help of a microcontroller.