If you double the input impedance of your power amplifier from, say, 30kΩ to 60kΩ you’re not going to hear a difference.
Yet, double the input impedance of your phono amplifier and you’ll hear a change.
What’s different between the two?
In the first case of doubling the amp’s input impedance, we won’t hear any difference because nothing in this chain affects frequency. The amp’s input impedance must be high enough to not load down the preamp, but aside from that, not much else matters.
That’s not true when it comes to a device such as a phono cartridge. Here the small coil generating the voltage feeding the phono preamplifier is part of what we refer to as a tank circuit—a tuned inductive network where the frequency response is a function of impedance and capacitance.
Think of it like a filter where the resulting output is dependent on the values of the elements that make up the network: coil, cap, resistor.
It is natural to assume that if the impedance setting of one element within our system matters, then it stands to reason all must.
Hopefully, it helps to have a short little explanation like this to set the record straight.