The 10dB drop is per channel. When they are summed, it typically returns to a net 0dB loss. It is standard when summing a stereo source to mono to drop the inputs down some so that the resulting output is the same level that it was before.
In the end, this is all overcome by just turning things up or down a bit if you need to, but I think I am correct on how a stereo mix to mono is typically treated.
You should hear very good audio on the output side without any change to volume when compared to a single channel driving the same audio. The difference will be that now it is a properly summed stereo signal. Good luck with all of it and let us know how it goes. I live in a slightly different world when it comes to this type of gear and I use a digital signal processor (DSP) for most of the mixing I do. But, I've used stuff from RDL and others at times over the years with good results.
