1. The formula for calculating filter frequency in Hertz is always 159155/RC with R = resistance in ohms and C = capacitance in microFarads. So a 1uF cap into a 22000-ohm load = 7.2 Hz. If the load resistor in the SE-120's circuitry is first, calculate directly with the above formula. If it isn't, you'll probably need to add an input-load resistor to use with the filter cap.

2. When one is replacing a higher-order high-pass filter with a lower-order one, one needs to use a HIGHER, not lower, crossover frequency in order to block sufficient lower frequencies from the midrange/tweeter. Personally, I'd start with a 3KHz filter and see how it sounds, especially at the highest SPLs you expect to use. If you're not overdriving the tweeter, add capacitance gradually to decrease the filter point.

But why not just use the input terminals and filtering in the speaker's crossover? Unless you wire around it, you're using the crossover's filtering anyway, and adding another filter ahead of the amp multiplies capacitor colorations and phase errors. IOW, drive both amps full range and let the speaker's crossover do the job it was designed for.

Or am I missing something?