Hi All
Don't get confused by the Mackie example. It just showed that externally mixing the separate D/A outputs improved the sound and gave a convenient way of A/Bing this. And then prompted the question of how good would this sound if the Mackie wasn't inline.
All pin 1 XLR or Screen connections on TRS are used for screening and can be connected together but you will find they are probably connected internally in the D/A so you will only need one screen connection to link the ground of the A/D to the what ever the passive mixer left and right are plugging into. Resistors should be 1% tolerance types to keep the balancing accurate. Also the passive mixer can be used with as little as 2 (2L & 2R) summing points to about 32 (32L & 32R). If you only sum 2 outputs the signal will drop 6dB and will drop 6dB for every doubling of outputs so. So up to about 8L & 8R running at pro output level can be summed and run back into to A/D inputs switched to -10 non pro or DAT input etc without any makeup amp. Or if you want to mix 32L & 32R use two 1272 modules and you basically have a Neve mixing buss but running fully balanced. Only inputs to the summing mixer that are plugged into outputs of equipment or shorted to ground count in the summing loss.
2 outputs summed = 6dB loss
4 outputs summed = 12dB loss
8 outputs summed = 18dB loss
16 outputs summed = 24dB loss
32 outputs summed = 30dB loss
Hope this clears some things up. I have only just become aware of this forum so I have not read the whole thread. So if I missed any questions feel free to ask:-)