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The reason you want to use a high resistance in the network is that the stereo separation depends on low source impedance and high mixing impedance to keep the source channels separated. There will always be some mixing with these resistive networks and it becomes a matter of experimentation or calculation to tradeoff the signal attenuation in the mono channel against the loss of stereo separation in the stereo channels. Higher mixing resistance equals more separation but less mono signal. A quick off the cuff calculation shows that with 100 ohm source Z and 47 K mixing and mono load resistances the stereo cross channel mixing will be below -57 dB. This also shows that the attenuation of each stereo channel signal into a mono load of 47 k will yield a loss of 9.5 dB for each channel but since the two channels add in the mono circuit the overall loss is 3.5 dB. This can easily be compensated if the mono circuit has reasonable gain.
This does not apply to a 600 ohm balanced system in which case a new set of calculations would be needed and it is my guess, without having done the calculations, that if the input and output Zs are 600 ohms, the mixing vs. mono signal loss will be unacceptable.
My experience is that using a sound card and 47 k mixing resistors in a single ended low output Z, high input Z system works adequately. Your results may vary.