They need to be custom made based on the input and output impedencies of the devices on either side (preamp and amp usually). If you can get those measurements (usually in the docs for the devices), folks here can give you direction for how to get the right capacitor values. Then you just wire a junction box with the capacitor in the path.
Not quite,
Inline filters are based in the
input impedance of your power amplifier or monoblocks. (Assuming you have separate amps and preamps)
Once we know that information, along with where you want the roll off to begin(between 40-80Hz), and if you are using RCA or XLR connectors, we can help get you started on the cap values that will be necessary for your system. (if you change amps in the future, the filters will also have to change unless they have the same input impedance)
if you are using a AV Receiver, most of them have a built in high-pass filter by setting the speakers to "small"
if you have an integrated amp, it's often not possible to implement a high-pass filter.
