Most solid-state audio devices as an opamp as the output, and they can usually drive any impedance greater than a few Kohms. In almost all cases, devices are designed to drive an impedance "much greater than" their own internal source impedance - they are happy as long as the impedance is high. And for solid state, 10K is usually quite high; the difference into 100K or 1000K is very tiny.
Tube gear, on the other hand, often has a hard time with low impedances. The classic RIAA phono preamp from the back of the RCA tube manuals calls for a minimum of 250K. But most tube gear is happy with 100K. We use 250K in the power amps, and 100K potentiometers in the amps with volume controls. Foreplay is 33K to 50K depending on volume setting, this is the recommended minimum load for a Seduction (though it is quite well behaved down to 15K).
So much for the minimum input impedance. For volume controls, at intermediate settings they increase the apparent source impedance of whatever drives them. The worst case is at -6dB, where even with a very low impedance source, the volume control adds 25% of the potentiometer resistance. (That's half the pot resistance to ground and another half in series, and as impedances the are in parallel.) This can have two bad effects if it is too high. First, it interacts with the input capacitance (Miller capacitance if it is a triode). The Miller capacitance of a 12AT7 for example is about 90pF, which gives a treble rolloff of 1dB at about 35kHz; it's -3dB at 70kHz. I don't like to go any lower for that corner, so I have to use a maximum 100K pot. Second, a high impedance point in the circuit is sensitive to picking up electrical fields such as hum and radio signals. You don't want that at the input of an amplifier.
If there is no level control or attenuator at the input, then the resistance to ground can be quite high. For tubes it is set by the tiny possible grid currents, and is usually specified in the tube manual - 1 Meg is a common specification. I don't like to go that high if I can help it, since modern production tubes are not as uniformly reliable as old stock from when they were made in the millions; I usually use 250K in power amps. In Seduction I had to go high in the second stage (it's 1 Meg) but I used fixed (LED) bias to make sure the grid current would always be small. And I measured the grid current on a bunch of 6922/6DJ8 types to make sure!
