Can you Try em and listen for yourself?
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.
Gabuli, sorry, but I'm having a little trouble understanding your last question. Let me try anyway to offer some helpful thoughts. The Sophia's are 87dB efficient with a minimum impedance of 3.1 Ohms. The Plinius is rated for 220 W class A into 4 Ohms, so I would expect it ought to be able to drive and control the Sophias without difficulty. The Cary's deliver 1000 WPC into 4. You have a bit more power with the Cary, but it is class A/B power. I have not heard the Plinius 103, but it is a well regarded amp. As much as I like my Carys, it is really hard for a class A/B amp to play in the same league as a good pure class A amp like the Plinius.
This is why I questioned if the unpleasant sound might be due to your Cary CD player. I haven't heard that player, but someone whose ears I trust told me that "Cary should stick to tube amps" after an audition with the 306 Pro.