rel subs using amplifier speaker outputs for their high level connection
it is the better way to do it anyways
The cap provides a first order high pass filter. You want to use a cap which provides the same impedance as your amp’s input impedance. So if the amp is 47k your cap should also be 47k at the crossover point, which I recommend no lower than 80 Hz. With those low efficiency speakers this will greatly reduce distortion and make your amp play much louder.
I believe this just happens to be 0.047 uF for you. I recommend Jupiter copper foils.
This simplifies subwoofer integration, and will double the apparent size of your amps. Also, most people report greatly improved soundstage and reduced distortion this way.
PS - There is a myth audiophiles have that they should buy a subwoofer only for 1 octave. Every single time I've convinced those audiophiles to try the alternative they never go back, not just for the bass but for the overall improvement in detail, soundstage, and dynamic range.