I have actually attended an Accuphase-B&W show organised by Alpha Hi-End in Antwerp a few years ago where they have compared the B&W 800 diamond with the B&W 802 diamond speakers using the following Accuphase set-up: A-45 power amp, C-2810 pre, DP-700 cd player and the PS-1210 power source. I also went the next day to the shop and listen to the system for another 3-4 hours all by myself.
I'll start by saying that Accuphase works very very well with B&W speakers. When driving the B&W 800 diamond speakers the Accuphase combo mentioned above outclassed many much more expensive systems that I have listen in that room (e.g. 80-100k full MBL system and a 200-250k Gryphon system.)
Regarding your question about B&W speakers, to my ears the 800 diamonds were much better than the 802 diamonds. However, it is very possible that the dealer optimised the position of the 800 speakers in the room while the 802 where place next to them in such a way to not stand in the way much. (During the show and also the next day the two sets of speakers where placed next to each other in the same room).
Regarding the capabilities of the A-70 to drive the 800 or the 802 speakers IMO they will be more than capable. The A-45 drove the 800s to ear bleeding levels, its meters went quite a few times into the "red zone”, but it never shut itself off. However, as I have said many times here, if you listen to large big-bands or orchestra, you will be better off with one of the Accuphase class A/B power amps. You won’t be happy if you will see the A-70 power indicators constantly going into the “red” zone. The only way to know this for sure is to try the combo in your home and with your music - in the store we all have the tendency to play the music much louder than we actually do it at home.
Regarding Toddmorr’s comment above, IMO the Accuphase will not burn the B&W tweeters no matter how loud you will listen to them. As far as I know they are design to prevent this, and will shut themselves off before doing this. Of course, this is my biased assumption. Tod might have burned his tweeters with an Accuphase amp.
Finally, why do you plan to buy the C-2420 and not the C-2820. In my experience, the C-28xx models are significantly better (and more expensive of course) than the C-24xx ones. If you have compared these two pre-amps please report back.