I've extensively auditioned the 6003, had the 650A for a month, and own the C326BEE. I've also home auditioned the 685s for a week when I had a C325BEE, and I've heard them with the Marantz 6003. I haven't heard the Pioneer amp so can't comment on that.
The 6003 is a very good amp for simpler acoustic music and sounds wonderful with intimate singer-songwriter stuff, small-group jazz, and chamber music. In my experience it starts to fall apart when things get more complicated, partially because its bass can sound very slow and plodding. Playing the first 'Imagined Village' album, the complex combination of acoustic and electronic textures tripped it up quite badly, as did the scale of Shostakovich's 5th Symphony. It is beautifully built for the price.
The 650A is a step up from the Marantz in terms of detail resolution and its ability to keep control of large-scale music but it loses some of the Marantz's musicality. In particular it has a tendency to sound overly bright and analytical in the treble and overall it can sound too clinical so it needs careful partnering to get the best out of it.
The NAD sits more comfortably between the two, with more control and clarity in complex music than the Marantz, but without veering towards the brightness of the CA. Of course, it doesn't resolve details as completely as the CA, but it is still a detailed listen and there has to be a trade-off somewhere. Its bass performance is far superior to either of the other amps but, unlike some earlier NADs, it is very well balanced across the whole frequency range. IMO it would power the 685s better than the Marantz or CA amps but I would also look at speaker options from Monitor Audio, Dali, and Q Acoustics.
The other amps you should look at are the Rotel RA-04SE and the Rega Brio 3. The Brio 3 works very well with Rega's own speakers, whilst I'd be inclined to try the Rotel with MA RX series and the Quad 11Ls.