[DaveC[_2_]]

I want to connect the audio out (headphone jack) of 2 computer sound cards to
a desktop & woofer speaker-amp combination. (The original input was via USB
only and I'm modifying this for analog audio input.)

This is my guess at the necessary resistors to mix down these 4 outputs to
the 3 inputs in the amplifier (L, R, sub):

http://i41.tinypic.com/97mpud.jpg

The data sheet for the Philips TDA8510J amp IC shows an application for 2
channel input:

http://i40.tinypic.com/5ets9w.jpg

Suggestions welcome.

Thanks.