[Senpai3330]

I read the "Wye" Not Rane note on summing boxes. The diagram above is taken directly from their discussion of a stereo-to-mono summing cable.

What determines the resistor value that connects the +signal to the signal ground? And honestly what determines the 2 resistors in the +signal path?

I will be using it to sum stereo preouts to a single LFE input on an active subwoofer.

 

[Armaegis]

The exact values aren't terribly crucial. The resistors on the + path are just to prevent your L and R from shorting into each other. Anywhere between 100-1k ohm is fine.
 
The resistor to ground isn't entirely necessary, but is an extra "foolproof" step . To prevent too much loss, that resistor (or your input impedance, whichever is smaller) should be at least an order of magnitude bigger than those on the + wires.