Hacking audio amplifier

A set of Harman Kardon Soundsticks are obsolete with most modern computers. They originally took digital audio via USB output from the computer and fed
it to a multi-channel audio amplifier chip which drove 2 desktop speakers and a sit-on-the-floor subwoofer. USB drivers are no longer available.
I'd like to make these work again by connecting the audio-out jack from the computer to the appropriate points on the Soundsticks' PCB.
The main component on the PCB is the amplifier:
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http://oi55.tinypic.com/egrdyg.jpg
My question is basic: how do I connect to both stereo inputs (inputs 1 and 2) and at the same time connect to the subwoofer input (inputs 3 and 4)?
I presume the answer is a simple resistor network mixing the 2 signals for input to 3/4?
And yes, there's filter networks there, too, and it is important to know the appropriate "injection" point, which will take some time.
Thanks, Dave
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Think a bit here -- the rationale for a common woofer is that the low frequency sound is common to both left and right channels. Therefore, all you need is a low pass filter from either channel 1 or channel 2. The input impedance of the common amp has to be much higher than anything you could make with resistors.

Assuming there's a low pass filter circuit on the board somewhere, connect channel 1 to its input.
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yes, plus a capacitor from 3/4 to ground so the sub only sees low freqs. Ideally this RC filter should match the speaker's characteristics, but jsut get it running now and you can tweak values later if needed.
I'd use as high Rs as is workable to minimise crosstalk.

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...in what configuration? (my main question)

What size?
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