The Cambridge 640 is nice but the built in Denon may simply be a better match. Nothing wrong with that.
As far as amps go, nice little speakers but fairly inefficient(86db). If your room is medium to large in size or you like to rock, stay at least at 60 watts/channel. Keep in mind that you have to double your wattage to go up 3db. What does that mean-by going from 60 watts to 100 probably won't be an audible difference.
Personally, I'd go for quality of quantity. What is your budget? Room Size? Do you like it loud?
As far as amps go, nice little speakers but fairly inefficient(86db). If your room is medium to large in size or you like to rock, stay at least at 60 watts/channel. Keep in mind that you have to double your wattage to go up 3db. What does that mean-by going from 60 watts to 100 probably won't be an audible difference.
Personally, I'd go for quality of quantity. What is your budget? Room Size? Do you like it loud?