I just researched this topic yesterday. The output impedance of the preamp should be within 10:1 ratio to the input impedance of the amplifier. Thus, the preamp is the lower number in the equation in this example. I have a solid state amplifier which provides 50k ohm input impedance. My tube preamp provides 592 ohms output impedance. Given the formula above, the two units are compatible. Hope this helps!
