Vandy sub owners-crossovers for 2WQ

Given a amplifier with 50k input impedence and Vandersteen's recommendation that the filter be set at the next lowest value ( according to the WX-2 box )which is 33K, my question is: If the amplifier is an actual 50K input (as is the case with FET transistors used in the input circuitry)why would you want to set it below the 50K? The instructions say set it for the input impedence---very confusing)If you want a 80hz high pass wouldn't anything but a 50K alter this? Also, does anyone have the formula for computing capacitor value for various input impedences to achieve a 6db high pass @ 80hz? Thanks