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In case anyone interested, this is how I ended up convering a STF-3 sub to support a subsonic filter with a cutoff of 20Hz instead of 25Hz, as shipped from the factory. More on the HSU Research forum.

NOTE: Do this at your own risk, I do not take responsibility for property damage or injury resulting from this mod. Lethal voltages are available inside the sub (especially on the BASH amp), so take good care and unplug the AC source before venturing inside.

As it turns out, the STF-3 pre-amp board is already set up for a switch, as seem in the following pic (area of interest is circled red):

(also available as an attachment)

Also inside the circled area is the resistor responsible for the subsonic frequency cutoff, Ry.

This is a crude schematic I drew while tracing the circuit around the missing EXTENSION switch Sy. It is a classic example of 12 dB/oct Linkwitz-Riley highpass:

(also available as an attachment)

Through experimentation with a signal generator and a scope, I have found that by substituting the existing resistor Ry of 0 ohm with 100 kohm, the frequency at which the bass boost begins falling off is at 20Hz instead of 25 Hz. Also, I found out that for a given input signal the max voltage gain at 20Hz is 1.35V compared to 1.15V at the drain of the FET in the above circuit. This means that the max low frequency boost is raised by 1.35/1.15=1.174 or 17.4% from 25Hz to 20Hz.

Some math to back it up:

Original Fs with Ry=0 kohm:

Fs=1/(2*pi*sqrt(239000*20000)*1e-7)=23.02Hz

Modified Fs with Ry=100 kohm:

Fs=1/(2*pi*sqrt(339000*20000)*1e-7)=19.34Hz

ANOTHER Fs with Ry=80 kohm (harder to find than 100k, but it gives a Fs very close to 20Hz):

Fs=1/(2*pi*sqrt(319000*20000)*1e-7)=19.93Hz

I did a couple (crude) sonic tests, and the output at 20Hz was around 4dB higher, while the output at 16 Hz was around 6 dB higher than with the original circuit. The measurements were taken with one of the ports plugged up. The driver movement at 16Hz is much, much more extensive with the 100 kohm resistance, so take care while testing this mod.