Hiss is generated by active circuits, and normally occurs in the preamp (input) stage of the amp.
The level of the hiss depends on both the gain setting of the amp, and the impedance of the source.
The source in this case is the bass. The tone pot affects the bass output impedance at high frequencies
If everything is working normally, the gain of the amp may be set too high. It is basically a matter of
signal to noise ratio.
Is the bass ouput level good? Are the volume pots set reasonably high? And the pickup height set
If the bass output is weak, the amp gain will have to be set abnormally high, and the input noise (hiss)
will be amplified.