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I can see a 4dB level discrepancy between two ISA 220s with ADC cards. What is wrong?

This could be caused by the 20/24dB switch on the ADC card of one unit being set differently to the other. This switch alters the dBFS calibration so that 0dBFS can be equivalent to +20dBu or +24dBu. This can help to prevent overload or under-recording on the receiving device.