They should not be equal. If the input/output impedance of a preamp to amp are equal, you have a maximum power transfer between the two. This is nice, however since each component has its own power supply, it's a useless nice. What is important is that the preamp's AC signal voltage is preserved as much as possible to the amp input. So, by increasing the amp input resistance with respect to the preamp output resistance increases the input voltage at the amp (voltage matching). The impedances are in series; so a larger Z differential means less current, less power but more voltage by simple Ohms law. The power suffers, but so what - with an infinite input Z the input voltage approaches the driving voltage. That's what an amp's input/driver stage looks for - as high a voltage signal as possible since it gains up the voltage, not the power.
This applies to balanced as well as single-ended signal transfer.
This applies to balanced as well as single-ended signal transfer.