I've got an old 4x12 that's loaded with V30's. At some point several years ago, somebody splashed some soda on two of the speakers. Is there anyway to clean the cones without damaging them?
the cones are treated wood pulp (paper). Nothing you can really use without risking damage to the cone. maybe there's some obscure chemical treatment out there - I don't work in the paper industry - but you'd be better off leaving them alone.
For dust I find a 1.5" paint brush works well. My PA speakers get especially dusty/dirty due to the relatively large holes in the grills. The horns collect a lot of gunk as well.
I would use a large, dry, stiff paintbrush. If that doesn't do the trick to get all the dust out, especially off the doping around the edges, I'd use a low-power shop vac with a brush-nozzle on the hose. I did just that the other day to some old Celestions and it worked perfectly. Just move it around in a gentle manner on the cone and the edges. Just dont' touch the center coil cover with it, though.