The playback engine on MIX (either 32 or 64 voices) only determines how many DSPs are being used to provide disk tracks into the TDM system. These "engine" DSPs (either 1 or 2 on a MIX system) don't do any mixing; they just provide a way to get the disk tracks (either 24 or 16-bit, depending on the bit depth you have selected) into the TDM system. Then, the "Mixer plug-in" is responsible for summing signals to any pair of outputs.