tubes and moisture

The tubes shouldn't be a problem, but excessive moisture can wreck a lot of other things. Thorman's suggestion is sound. This may seem counter intuitive, but an air conditioner works great for keeping the moisture levels down. The heat goes outside instead of indoors as with a dehumidifier

Lapierre, Excellent suggestion. From everything I have read the best all around humidity is 45-50%. 35% is too low and 55% can be too high for certain instruments like a piano. 45% is a good number to shoot for. Its good for you, your house, any musical instruments you might have and your stereo equipment. I have a humidity meter in my listening room and it is usually between 45 and 50%, but I live in a dry climate.

Corrosion in your voice coil gaps, tube sockets and potentiometers, and deterioration of foam speaker surrounds, would probably be the most worrisome effects of high humidity. Then there's the possibility of corrosion to any other contacts, that are not plated, in the system. A good dehumidifier, or an AC unit, should be left on at all times, except when you're listening.

If you are using a dehumidifier you will be fine.