The PCB holds two MFB filters based on the Aikido gain stage. If a two-way active crossover is needed, then two PCBs must be used.
The punch line is that the MFB filter is a better active filter, which explains why it is often used in demanding audio applications, such as high-dynamic-range ADC input stages. It is slightly more complicated than the equivalent non-inverting Salen-Key filter, so it is largely unknown to most audiophiles.
The multiple-feedback filter is based on the inverting amplifier topology, whereas the Salen-Key is based on the non-inverting topology. This makes a big difference, as the assumption behind the Salen-Key topology is that the buffer's output actually realizes absolute unity-gain; no buffers do. In contrast, the multiple-feedback filter relies on there being phase inversion of the input signal, which there always is. In addition, the MFB's initial RC filter ( are outside the feedback loop, so this pre-filter is essentially passive, therefore incapable of overload.
The Aikido gain stage offers low distortion and low noise with a low output impedance. The best input tubes are the 12AT7/ECC81, while the 6DJ8/E88CC works best as the output tube.
The part kits include the capacitors and resistors (with many extra values) required to populate the PCB, but not the filter portion. The low-pass filter requires three resistors and two capacitors; the high-pass filter, two resistors and three capacitors. Each PCB hold two 2nd-order filters, so two are required for stereo.
PCB includes 16-page user manual.