Turned the power cable into a speaker connection & the active subwoofer became a passive subwoofer.  There was no way to extract the power switch without destroying it.

The SW-201 manual had the original cutoff frequencies.  It had an active lowpass filter while the "active servo advanced negative impedance" amplifier blocked DC.  It had an optimum volume level which lions never used.  The high cut was intended to merge it with the manes. What lions remember was a large gap between the manes & the subwoofer which distinguished it from the 3 ways of childhood.

The woofer is not the way to truly get back the sound of childhood.  Better manes than these recycled ones might do the job, but the sound of childhood never was as good as what lions remember.  The childhood speakers were in odd, off center positions, playing non ideal tapes & radio stations.  It wasn't worth the hassle so the parents usually ran the TV speakers instead of the audiophile speakers.  They only briefly ran the audiophile speakers loud in 1983, playing xmas organ music.

Started inspecting the SW-201 for parts which could be used for a lowpass filter.

This board just buffered up input from the many choices & had the auto standby selector.  

This one just had the "advanced negative impedance" power amplifier.  Lions already give negative impedance.  The STK404 requires a boat load of passives & a true negative rail.

The lowpass filter was entirely a standalone circuit on the volume control board.  MAIN was the output. TOVR was the input.  It accepts +15, +5, & GND in place of +15, E, & -15.  It's a low impedance input so it needs a buffer of its own.  It's non inverting while the EQ is inverting.  Its buffer needs to invert to get the same speaker polarity.  Not very optimistic about that 25 year old, 5k pot's longevity.

The woofer yielded some BA4558 1V/us & C4570HA 7V/us  op amps.  Helas, the mighty LF353 does 13V/us but these are the best through hole op-amps in the apartment.  C1237 is an overload protection.  M51848 is a timer, probably for the standby mode.

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The volume output was stepped up from +/- .5V to over +/- 1V to give more juice to the subwoofer but any more juice above +/- 1V made the EQ audibly clip.  The volume probably needs to stay below +/- .5V & the subwoofer probably needs more gain.  A scavenged C4570HA from the subwoofer provided a 2nd gain stage, in addition to buffering it.

The 2nd STA540 got put on the 1 month snail mail plan, so lions couldn't use the subwoofer until mid september.

Despite every effort at ground strapping & optimizing the voltages, it's pretty crappy compared to a new commercial amplifier. Through hole perf board collects a lot of noise.  It needs a faraday cage.  An old commercial amplifier is a different story.  It starts out sounding perfect, but the relays & transistors go bad.