One thing to consider is that when a driver is treated, most people only treat the front. Unless the driver is removed from the speaker(how many people do that?), the surround and cone will still dry out. As you have butyl rubber surrounds, and I am presuming polypropylene cones, they should last a long time. And I question whether anything can be used to dress polypropylene, and have an effect. It is a very inert polymer. Unless the cone was plasticized(which I kind of doubt), putting any of these chemicals on the cone does nothing, except make it look wet. By the way, platicizing is the process of mixing a very high boiling solvent in with the polymer during manufacture. It makes the polymer softer, and pliable. The solvent(plasticizer) takes years to evaporate. Which leaves only the polymer(which was hard in the first place). That stiff, brittle polymer is prone to cracking. If the drivers are not of a common size/design, think about a having a replacement set on hand(kept in a good environment, where they will not degrade). That is, if you don't plan on moving to different speakers when these go anyway...
