Gabuli, I have owned Cary 500.1s for about two years, using them with Magnepan 3.7Rs. I wouldn't call the Cary's slow. I could imagine them being called sweet or warm. They control the Maggies well, so I suspect the Sophias will be well controlled. By the way, I very seriously auditioned the Sophia 3s when I bought the Maggies. I liked the Sophias very much, and they gave a very musical presentation with dead accurate timbre. They were driven by a solid state BAT amp/pre with an Esoteric source, which seemed to do very nicely. I would think the Carys would do well, but I would have thought the Plinius would have been a great choice. They lack of control would seem to be the amp, but I wonder if the unpleasant sound is due to the Plinius or the Cary 306?
At any rate, if you are able to find a used pair of Carys, I'd say go for it. If you are talking new, that is an expensive experiment. I'd be more inclined to rotate used amps into your system until you find something you really like. You don't loose much on resale.