Ayre has a certain darkness, like it pulls noise out of the room. It's kind of a funny effect. Very smooth, touch warm in the sense of Parasound.
Luxman is closer to D'Agostino. Extended on both ends, liquid midrange.
Have never heard Simaudio so I can't help you.
The Luxman/D'Agostino/Ayre triad represent amps I'd be happy with for the rest of my life.