The sound of nuendo15 sounds brighter and closer, and it doesn’t feel as spacious as 13. Does anyone else have this feeling? Has the official modified the sound quality?
I can’t speak for Nuendo since N15 is my first version on it, but if I open a C13 project in C15 or N15 it sounds identical here.
Hi and welcome to the forum,
if this was the case the forum would be flooded all over with reports. Do you have any plugins in Control Room by any chance? I would investigate in that direction if I were you 
Also a classic: having control room activated, but still the main outs in the Audio Connection Outputs set to your audio interface ports…