Epilogue on all this: We had a pressure regulator put in, on the main water line coming into the house, just on this side of the big shutoff valve for the whole house, in the crawl space.
Before that, I installed a mechanical clock timer on the humidifier, so that it can only run for 30 minutes out of every 2 hours. That way it doesn't run for hours at a time and over do the humidity and cause stacking.
Between these two steps, so far so good.
The house pressure is now around 70 PSI instead of 80-100 and the toilets take a little longer to refill.