Impedance matching is a rather vexed question - it depends a great deal on the situation.
Strictly speaking we match the impedance of the load to the source to achieve maximum power transfer.
In the case of valve (tube) amps the high output impedance of the power amplifier valves is matched to the much lower speaker impedance using a transformer, and in that case something close to an impedance match is sought.
With the line voltage level connection between preamp and main amp it is voltage not power that we are trying to transfer and therefore no attempt is made to match impedances. The output impedance of a preamp will be low, hundreds of ohms to ohms, and the input impedance of the main or power amp will be very much higher, 10's of k ohms up.
These two impedances are effectively in series and divide the actual output voltage source, so making the ratio between them very high drops the least voltage.
Similarly where the output of a solid state amp drives a speaker the output impedance of the power amplifier will be a small fraction of an ohm while the impedance of the speaker(s) will be typically 8 ohms (but maybe 16 down to 4).
If we tried to actually match the impedance of the solid-state output stage for maximum power transfer, half the power would be dissipated (uselessly) in the output stage electronics as heat, and the other half in the speaker(s). Typically this would also grossly exceed the current capacity of the output stage and destroy it.
Impedance mismatch is actually used all the time to limit the available power, and no attempt is make to match the impedance of your toaster or electric jug to the power station because, if it were, you would get half the power station capacity appearing in your kitchen - both spectacular and unhealthy.
So these appliances are constructed with a deliberate and gross impedance mismatch to the power station so that they only dissipate a few hundred watts of the megawatts the power station is churning out.
In radio systems, such as CB antennas, the object is maximum power transfer, voltage and current, so the antenna is matched to the impedance of the transmitter output stage.
@Onewatts - this is another case again, where the multi-effect is used mainly for signal conditioning, and PA system inputs actually have much greater voltage sensitivity than your typical guitar amp input.
You would consider this a preamp in the guitar amp sense of the term of lifting signal voltage level if it only fed a power output stage, say via Fx Return, but very generally most Fx units won't produce the sort of voltages required by most main amps for full output (e.g. 100mV v 1 volt).