prevent hardening of speaker surrounds

As I state above, my feeling (and Dynaudio's) is that Armor All has no negative effect on a butyl rubber surround, or the glue that adheres it to the cone. I made no comment at all on its effect on a foam surround, and can only guess that it could very likely have a negative effect on foam. I also never said that the surround has "little effect on the sound", as you imply, I just said that the spider has vastly more. This assumes that the surround appears in good working order. If it is not, of course there will be a very noticeably degradation of performance. However, I never implied the contrary...If a surround can't keep the cone perfectly centered, of course there will be all sorts of bad things happening. Of course it is necessary to do whatever yopu feel is called for to maintain it, and I hope we all feel this way.

Stiffness (compliance) of a surround will partially determine the speaker's primary resonance (Fs), so it stands that altering the surround will possibly shift its resonance. It's my understanding that Armorall is a monomer dispersed in an aqueous (H2O is of course Earth's most prevalent solvent) solution, and its action is to deposit a one-molecule thick "preserving" layer on vinyl, and possibly butyl, as well. The fact that you can see it on butyl surrounds after it dries lends me to think that it's helping to seal them as it does very well on vinyl and rubber auto door seals and trim. Trouble is: how do you treat the backside?! Maybe the relative effects of spider-hardening vs surround-stiffening are shifted as a function of ratio of cone/voicecoil diameters, too: I can imagine that the quality of the surround of a 12" woofer is relatively more important than the aging of its spider, whereas the reverse might be true of a 4" midrange with a relatively stiff surround to begin with. Just conjecture, though. In the past I have run into high-tolerance voicecoil-gap drivers that scraped after a few years, probably due to spider "sagging". Running the woofer upsidedown for a while usually remedied the problem. So gravity figures in this, too!

Interesting...however, I don't see the spider as ever "hardening", I see it simply wearing out; i.e., the fabric strands develop tiny fissures that grow and become more numerous with time and wear through usage. In effect, they become "threadbare". I also agree that the effects of gravity cannot be ignored.