>
> When you connect a multi-track tape deck's outputs to the track inputs
> of a mixing desk, don't the signals pass through all that complex
> processing? Therefore, a plausible DAW implementation could attempt
> to do something more than A+B=C.
>

Of course it's possible and some companies do. But generally if they do they will model it on a well known and loved analogue desk and have it plastered all over their advertising in big letters. For example Propellerhead's Record which has a channel strip based on SSL.

But again, at unit gain input and all EQ flat even desks like this will be designed to be as flat and transparent as possible. There will always be some noise, there will be some kind of analogue compression on overdriven signals (rather than digital distortion of integer math or infinite headroom of floating point math) but the main difference will be it's a channel strip and the sound of the EQ and Dynamics (compressor) which are included in the channel strip, rather than the summing involved.