Treating High Humidity in basement

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.

Hi : I live in Massachusetts and have my second system in my basement and have a De-humidifier in the room with the system...I leave it on all the time from May - November...The only time I shut it off during the damp season is when I am down listening..After listening I turn the system back on..I try to keep the room Humidity about 40 % .. Any quality Dehumidifier in the room should work..Just make sure it is large enough to do the whole cellar...It shouldn't be a big deal.......

The floor heat system may be the sticking point. An appropriately sized dehumidifier works fine in our house, but we have forced air heating/cooling. During the cold months, the forced air heat dries the air so that the dehumidifier is not even needed. I am not familiar with your floor heat system (assume it is hot water in pipes, but could be glycol in plastic tubes?), but the temperature differential between the heated room and the surrounding ground will contribute to a migration of water vapor into the room. If there is not an adequate vapor barrier below the slab, you could be bringing significant moisture from the ground into your basement area. Moisture vapor will also migrate through the walls if they are not damp-proofed. I believe the best place to start would be to purchase a good quality, moderately large dehumidifier and let it run for a week or so then see what you think. It sounds like you own a hygrometer, so you can measure the relatively humidity at incremental distances away from the dehumidifier to check the effective radius of treatment. Depending on your basement layout, you may need two dehumidifiers. The next step would be to install some form of forced air ventilation system, but before doing anything like that, I would consult an HVAC contractor. In fact, I would probably call a contractor as soon as I determined one dehumidifier alone was not going to work.