The non-dithered stereo Mixer plug-in can sum up to 59 inputs to a pair of outputs on a single MIX DSP (68 x2 on the HD hardware) It doesn't know anything about where these signals have come from - they could be normal audio (disk) tracks or Aux inputs - any 24-bit source from TDM. There is no truncation until the final outputs (for example, the signal leaves the mixer plug-in at output 1-2 to go directly to the engine DSP connected to the 888/24 interface). In the dithered mixer case, dither is added at the final output, before truncating back to 24-bits.