Sdcampbell is quite right. In general, the higher the position of the volume control knob, the less negative influence the attenuator will contribute to the sound. (The best potentiometer is no potentiometer.) However, there were/are a few old preamps still floating around out there that used the "pot" in the op-amp feedback-loop to actually increase or decrease the gain of the circuit, not as the typical in-line signal attenuator. Thankfully, not many attempted this as over time the circuit stability and sonic performance suffered. Gunbei- I think you're referring to amp input and pre-amp output impedance, not sensitivity. A ratio of 100:1 is an ideal. In practice, if cables are kept short, I've had good results down as low as 15:1.
Enjoy the music!
Enjoy the music!