We did get a great new dehumidifier this summer!Good for you! See, they fit in the house!
Time to make that basement a listening cave!
I would think the weak HF on the one speaker is either a dirty pot like you mentioned, or a bad capacitor in the crossover. You could always swap tweeters to see if the problem follows. If the deoxit doesn't fix it that is.
Nice find
Cadmium powder is usually a yellowish powder. I can't tell from your pics if that is what it is.
If you are planning on keeping these speakers, I would consider replacing the pots with matching units, or just replacing them with fixed resistors.