Free speaker tweak that's a must do

Keep in mind you can over torqued them and they will just loosen again. I suggest thread locking compound if loosening on you. And the brass screw deal. The magnetic force is very very small and far to far away to have any effect on transducer. But if you feel this maters have at it just don't snap a fastener brass isn't so strong.

Most speakers should be tightened every 6 months or so depending on how loud they are played.

Only in case if you receive ones with ones that you can unscrew with fingers.
Overtorque may be worse than under.

I would suggest using a non-magnetic screwdriver. Some driver magnets are surprisingly strong, and can seem to reach out and grab a screwdriver, leaving a punctured driver in it's wake.

I never ever had a issue with it. And I've owned 100s of loudspeakers. Built many more. Most can and do play at SPL Polks could never approach. I do know once you've over torqued a fastener your going to have it loosen up and this is what Polk432 is probably experiencing.