Also, does your method require make-up gain (i.e. running the summed XLR outputs to pres then to the A/D)? That would be cool.
Any passive summing network loss will be dB=20*log(1/n), where n is number of channels. Whether or not makeup gain is required depends on your setup. With 4 channels, the loss is -12dB, so for example I can run my D/A at +4dBu, and A/D at -10dBV, and no makeup is required. Also if you are running into a compressor, it shouldn't be an issue.
edit-- I see; you need to use either a BTA or some inline pads if you want to run them into mic pres?
Maybe; depends on the pre's ability to take high levels. But I wouldn't select a transformer over a pad unless you want the transformer's sound (and it will muck with your audio a bit, it is not transparent nor intended to be), or unless you are doing a balanced/unbalanced conversion.
So, if you were summing four stereo stems/busses, you'd need two Mix-4's, which could be run directly into a converter (with an XLR to TRS cable or adapter if required)
If you have 8 channels of D/A, one thing to consider is whether you want to run 4 stereo stems or 8 mono stems, and do left and right separately. If you are strictly doing summing, or even going to a mic preamp, you should be able to sample-align left and right sums done individually. If you are going to a stereo-linked compressor, obviously that doesn't work as well.
and if you wanted to run the Mix-4's into mic pres to color the sound, you'd need either the BTA's or a pair of 12dB pads. Is there a loss of fidelity or increased noise by doing that? It sounds like the BTA might be less noisy?
If you add more stems, then you have greater network loss (24dB for eight channels), and that might low be enough to feed the preamp.
With passive devices the only noise is thermal, which is down in the -130dBV range. You can also pick up some transmission line noise, and of course the self-noise of the outboard gear (you should be hitting it pretty hot though) but I presume these are the desired qualities of OTB summing
