Hi,
As you mentioned the SR10 has more features such as fm radio and sub out.
So depends if you require all the extra features now or maybe in future based on your setup.
Very often we go for a particular model having extra features but don't really need or use them at all.
So purely in terms of music reproduction, the 651a would be a better choice.
It has a better selection of internal components and does not have any additional circuits interfering with the sound.
The SR10 will also be good sounding. However the 651A would be much better.
Now between the Marantz and CA, we are talking about 2 completely different sound signatures.
I haven't compared them side by side but am familiar with the sound signature of each on various occasions.
The Marantz is much more musical with a wider soundstage.
The CA is a more direct and precise sound.
Other opinions may differ.
Try and listen to them if you can to decide which sound you prefer.
Do listen to them repeatedly if possible.
Also your choice of speakers would play an important role.
Hope this helps.