My graphics card is ATI; I do use color management software (Huey Pro) because I am a photographer. This creates a specific profile for the monitor which I use. (I did test calibre without the profile, but nothing changed.)
As I am thinking about this, I do wonder why the calibre GUI plugin needs 32 bit color depth. I could understand if there were photographs involved (perhaps for book covers?), but perhaps this requirement is a little overkill. Just a thought.