I want to route audio output from MacBook Pro to the line input of audio interface on another Mac

I have an old MacPro with a FireWire audio interface (M-Audio Profire 6). I have a main and a secondary mic attached to the two front mic inputs. I want to feed audio output from my MacBook Pro 2019 to the Profire so that all of my audio can be played over my connected speaker system. The only suitable inputs remaining on the Profire are the two “line inputs” on the rear. When I connect an audio cable from the MacBook Pro audio output to one of the line inputs on the Profire the signal strength is only strong enough to play audio through my speakers at a very low volume with the volume controls at max. I obviously need to amplify the output from the MacBook to make this work. What is the best way to do this? Thanks.

John