I wouldn’t call it mixing, it’s actually changing note per note, not only to create your own piano, but even to go as far as sound design. Couldn’t say it better than Brian Eno who apparently only recently discovered and got Pianoteq:
«I bought Pianoteq 6 a couple of weeks ago. I think it is the most well-realised and intelligently constructed piece of music software I've ever had the pleasure of using. It is an absolute milestone. Aside from its incredible power as an emulator of pianos, the possibilities that unfold when you start to edit the models are astonishing. I've been lost in it since I bought it, and keep dragging friends into the studio to show them this miracle. The randomisation possibilities are so intelligently constructed and are a great help in understanding the enormous variety of possibilities this instrument offer. I don't think I've ever been so impressed by a piece of software design and I just wanted to thank the team who put this together.»