Ath - my paper size is 8x10, printing a 6x6 negative. I've read in a few places that stopping the aperture on the lens right down wasn't a good idea - but I'll experiment with the one I've got.
Rick - I'll check to see if the D6700 does have attenuators - not sure that it does, but I'll have a look.
Willie and Bill - yes, a lower wattage bulb might do the trick - I'll check to see what I have in there.
Roger - thanks for the pointer about increasing ND by dialing in equal amounts of CM&Y.