Hi Guys !
I used to wonder about that myself until I tested it.
I'm using the same hardware configuration to run Cubase 4, AA 3.0, Nuendo and wave Lab with plenty of out board gear. I often track in any of these programs and transfer the files back and forth. If they are recorded with the same interface (SSL Mixpander) same, preamps, same set of converters and so on. It sounds the same.
For instance I track a vocal in in Nuendo, export it and insert in any of the above listed programs play it back save it export it and so on.. comparing the result file will sound the same if I drop it back in Nuendo and compare it to the original file side by side.
If you look at it this way, the analog signal is being converted to digital data and therefore it becomes just a binary code that is being captured by the DAW, if you use the same set of converters it will always sound the same as long as there is no corruption or digital sound processing alteration and capturing quality is kept the same.
Does that help ?