The RK6 is a TOTALLY UNIQUE product, one that has never existed before.  It is a Sallen-Key lowpass filter with a feedback loop, to allow it to resonate.  There are no integrated circuits or transistors in the signal path.  It is a PURE TUBE design, the sort of design that might have been invented in the 1950s if anyone had dared to create a turnable lowpass filter for musical uses before Moog and Buchla. It even uses a coupling transformer to provide the proper phase of the feedback signal -- pretty a "classic good old days" approach, and one rarely seen in the 21st century.  Two "vactrol" optocouplers tune the filter, and another coupler allows CV control go the filter's resonance.