I'll be adding information here from time to time about how to do some basic servicing of vintage JansZen speakers. More to come.
There are two cases where one might hear hum due to a speaker problem. One affects the entire speaker, and the other only smaller area.
Single woofer or small number of woofers:
If the hum is caused by a woofer panel failure, it's usually just one. In that case, the hum will only be coming from that woofer panel, i.e., isolated to one small area of the speaker. Removal and a rebuild of the woofer may be needed, but a thorough cleaning of its visible surfaces might do it. More about this when time allows.
Hum from an entire KLH Nine speaker is usually caused by a bias leakage path that demands current from the bias supply, The supply is designed to simply maintain charge on the membranes, and is unable to produce current. When asked to supply even small amounts of current, some AC hum will ride on the DC and this will be audible. If the leakage is very severe, the speaker will also be quieter than it should be.
Coincidence with the arrival of humid weather could be a clue that this is the problem.
This is not the same thing as an arcing path or carbon track. (But if you find a carbon track, do sand it away!)
Leakage paths will not be visible, except possibly as a build-up of soot. Such paths can form on the supply, the terminal block on the supply or speaker, or inside the speaker. The terminal blocks are easy to access, so . . .
A quick thing to try after removing the supplies is cleaning the terminal boards on the speakers and the supplies, as well as the wax between and around the terminal strips on the supplies.
Remove the seven banana plugs and lift the wires from the screws.
Wipe the block with a paper towel dampened with ammonia (non-detergent type) until no more residue appears.
After thorough drying, wipe with denatured alcohol (preferably from a hardware store, not rubbing alcohol).
In addition, the terminal block could be sanded lightly, especially importantly around each terminal, with very fine grit sandpaper. This will make sure that any invisible conductive path is disrupted.
A final step that I consider optional is to spray the terminal block with a light coat of Krylon Crystal Clear acrylic spray paint. Do not use any other type of paint, especially not anything labeled "lacquer." They are almost all slightly conductive and will make things worse.
After allowing the surface to fully dry, reattach the wires and banana plugs, then mount the supply. Hopefully, the hum will be gone.
The only B&M store I know that's left to buy the paint at is Sears or a Sears-associated hardware store. Art supply stores probably have it, though.
It's also available on line through Amazon and art supply stores like Blitsy.
There is a chance that the phenolic terminal block material itself has become slightly conductive. I would not have believed it possible, but we're finding this problem now on some of the Nines we're rebuilding.
In that case, the blocks must be replaced. The replacements should be made from G10 or FR4 (epoxy-fiberglass). Now these are made depends partly on whether you can get Kapton (or other polyimide) tape. If you can get it, then a counterbore in back isn't really necessary -- just use flathead screws and countersink 1/16" to 3/32" more deeply than necessary to recess the tops of the screwheads. Tape over the tops with a few layers of .001" or .002" thick polyimide tape. If you can't get such tape, plain electrical tape will do, but it's then critical that the screwheads be recessed by at least 1/4", in which case, counterboring and using panhead screws will be more practical, as with the originals.
If you are handy, you can order the fiberglass material, such as from McMaster-Carr, or obtain it from a local plastics supply house, and copy the original. Three inch wide strips are commonly available, and the extra half inch width is okay -- no need to cut it down to 2.5". If you're not handy, you can hire a machine shop to make copies.