Hi
@Andrei2! Welcome to ASR.
It's possible that the Marantz's Audio out ports operate at fixed volume and do not react to the volume control. Please try plugging something into them, a Bluetooth speaker, field recorder, or some other device with Line/Aux in to test this.
If the Marantz volume knob and remote control change the volume of the Audio out ports, then you can connect them straight to the KEF's Line input.
If the Marantz audio out ports do not react to the volume knob or remote control, then you have two options:
1. Connect the KEF's Speaker input to the Marantz's Speaker output, in parallel with your actual speakers.
2. Do Audio source->KEF Line in->KEF Line out->Marantz Power Amp In.
In that case, the Marantz would turn into a "dumb" Power Amp with just one input and no volume control. You'd have to control volume at the source instead.