I suspect your hum may be a result of the different signal ground / mains earth arrangement on the old Nait compared to the new. Sources fed from wall warts or with 2 pin mains connectors can cause hum with the later amps. As you don’t have a ground post on the 5si, try connecting a wire between the outer of one of the RCA sockets to mains earth and see if that gets rid of the hum.