FerdinandII wrote:
You need to just forget the concept of impedance when looking at this issue. It is irrelevant.
V=IRThe line level output is (generally) a voltage source.

The line level input is primarily straight resistance.
The only thing that changes when the input resistance of the following stage goes up or down is the amount of current that the (voltage) source device has to put out to maintain it's behavior as a voltage source.
Not quite true. The line output of a preamp or other input device to an amplifier input is not a pure (infinitely low impedance) voltage source. It may range from a few ohms to over 100K ohms. The input impedance of the amplifier is also not a true resistive load although it might be close to that. There is usually some capacitance and perhaps a very small amount of inductance at the input. That is why it is called input impedance, not input resistance. The effect of the input capacitance (and interconnect cable capacitance) reduces the delivered output voltage of a preamp that has a high output impedance. If the preamp has a high output impedance, the combination of that high output resistance interacting with the parallel capacitance of the cable and amplifier input result in a 6dB/octave low pass filter, much as what you would see in a speaker crossover. The crossover point and magnitude of that drop depends on the circuit values.
The net effect of using a device to attenuate the input voltage to the amplifier depends on a number of factors discussed above. However, deviations from the ideal components such as a potentiometer that may have some capacitance or inductance leads to a non-linearity in the ability of the device to attenuate the signal across its range. For example, if a wirewound potentiometer is used, when the wiper is at the top of the pot, ie the wiper is in contact with the output of the preamp, there is little if any series inductance between the preamp and amp to affect the signal. As the wiper moves away from the input side, in addition to a resistance increase there will be a series inductance increase. The impedance change vs the resistance change will make the pot act like a low pass filter. Add that to amp and cable capacitance, you now have a 12 dB/octave low pass filter.
You might say so what if the wiper is at the top of the pot. That depends on the values in the circuit. Even if the pot is a perfect resistance it will load the output of the preamp even if the wiper is not connected to anything resulting in some attenuation of the output of the preamp. Just for the sake of discussion, although very unlikely, let's say the inductance of the pot is very high in relationship to it's resistance. At low frequencies, because of the ratio of inductance to resistance, the pot itself will be lower in impedance, loading the output of a high impedance preamp more than a low impedance preamp resulting in essentially a high pass filter. Again, this results in a non-linearity of the attenuation of the supplied voltage vs frequency.
So, at the end of the day, unless there is an intervening gain stage, anything between the output of the preamp and the input of the amp will reduce the voltage to the input of the amp and perhaps not in a linear fashion. It is important to remember as Roscoe mentioned that the voltage output of the amplifier is only dependent on the input voltage and the internal gain of the amplifier, although the amp itself may have a non linear frequency response which is another matter. The takeaway is that any device that is between the output of the preamp designed to attenuate the signal should be as close to ideal as possible. That means when a pot is used, it should have low inductance, low capacitance, very linear change of resistance with movement, and the best compromise of resistance value taking into account the output impedance of the preamp and the input impedance of the amp. Also consider those pesky non linear elements like capacitance and inductance in the interconnects which may have a greater or lesser effect depending on circuit impedances.