Hmm
I've just downloaded the schematic from
www.radiomuseum and it shows an IC TBA570Q (IC 101) driving the audio output transistors. This IC is also used as am mixer and AM/FM IF amplification.
Pin 12 of the IC receives audio in from the volume control slider via C411 1uf; Pin 11 is audio out to T402/T403 audio output transistors. T401 sets the DC level via R416.
The DIN socket connection is from the top of the volume control and if the right volume level is there, then it shouldn't be too difficult to see why it isn't loud and clear out of the loudspeaker.
Chris nee Simpsons