Amplifier Input Impedance

If you use unshielded interconnects, then yes a high input impedance will be more susceptible to rf noise, but it's very unlikely you will hear anything, unless you live outside a taxi/cab rank, analogue cell tower, AM/FM radio tower or similar.
As we are at the low interference end of the system, it would be a different story if we were looking at the input impedance of say a phono stage or MC cartridge step-up device phono preamp.

Cheers George

Cheers George

The input loading resistor to ground cannot effect gain.
But in solid state bi-polar input dc coupled amplifiers, if it's raised too much with an open circuit input will effect the amount of DC offset seen at the speaker terminals, which could be out of reach for any dc servo's to correct. But once something is plugged into the input, (eg preamp dac etc) the output impedance of that device becomes the input impedance of the amp, and all goes back to how it was with a smaller input loading resistor.

(as for what varies gain in an amp, it's the feedback loop and values of resistors used that raise and lower the feedback.)

So in a tube or solid state (with fet input) you can raise the input loading resistor.

But in a solid state with bi-polar input transistors you can also raise it within reason 68k or even 100k, but you should never turn it on without anything pluged into the input. Otherwise you run the risk of lots of dc going to the speakers.

Cheers George