This sounds like a question for a builder
http://www.extension.umn.edu/distribution/housingandclothing/components/7051-04.html
As it says in the link using a dehumidifier may cause more problems as it will accelerate moisture ingress through the basement walls. Fixing a damp basement can be quite involved and expensive.
By the way, living in Boston MA we have 68%+ humidity throughout the house for much of the summer. I haven't noticed it causing premature aging in my hifi, and I actually like more humid air (as opposed to the winters when the air is bone dry). Provided there's reasonable ventilation and that mold is not present I don't see the problem with 68% humidity.
http://www.extension.umn.edu/distribution/housingandclothing/components/7051-04.html
As it says in the link using a dehumidifier may cause more problems as it will accelerate moisture ingress through the basement walls. Fixing a damp basement can be quite involved and expensive.
By the way, living in Boston MA we have 68%+ humidity throughout the house for much of the summer. I haven't noticed it causing premature aging in my hifi, and I actually like more humid air (as opposed to the winters when the air is bone dry). Provided there's reasonable ventilation and that mold is not present I don't see the problem with 68% humidity.