Hi Afj,
I've listened to both amps extensively, and both go decently well with the MA silver line. When Dinyaar says he's not a fan of either, you have to remember that he's used to Brystons which are in a different league altogether!
You haven't mentioned what your source component is - CA 840A is a very competent amp, but would not go well with overly analytical or bright sources - so pair carefully.
The NAD can certainly supply plenty good bass, esp. with those floorstanders, if it's not doing so I recommend taking a look at your source and/or speaker placement.
Compared to your NAD, the CA would sound more 'airy', the mid-range would sound a little thinner (perhaps that is what is meant by neutrality), while the bass would be tight and plentiful. Soundstage/ imaging would be better. Noise floor would be lower, especially if your system is fully balanced. You have to decide whether this is worth it for you. I guess the kind of music you listen to will also matter. Vocals are much, much better on NAD.
Having said that - both components being in the same category, it's still very much a sideways move, and IMHO you're better served saving up for a more substantial upgrade.
Hope this helps.
Regards
Persiflage