Amplifier with bass, treble and volume contol
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Preamplifier to power amplifier connection is simply a matter of connecting the outputs of one to the inputs of the other.

One odd thing is that the pots are wired backwards on the BA5406 schematic. Normally they are wired as shown in Figure 1 but this requires log pots (audio taper) so that loudness increases smoothly with rotation. The backwards pots may be a trick to use cheaper linear pots and still give an approximately log response. They actually short out the preamplifier when turned down to zero which is unusual but not dangerous.