It sounds like you have 2 things working against you xringer. The first is that your basement and water heater may be acting as heat sinks. The water heater for sure and the basement maybe. The other thing is the attic as source of heat might be insufficient. I'm not saying these things are facts but they are the most important to look into. The need for a lot of fan power seems to me to be a symptom and not a cause of the problems.
The problem with the attic as it is that the heat is diluted by the large volume of the attic. You only have so much radiation energy entering the attic. It might not even be that much in Massachusetts. If you don't restrict that radiation energy to as small a volume as you can reasonably do then the temperature will be lower and the volume of air that needs to be moved with a fan is much higher.
For example you might have 1000 cubic feet of air that is heated by the sun in the attic now. If you restricted that to 50 cubic feet through some kind of insulation then you would need a fan that moves 50/1000 or 1/20th the amount of air down to your basement. That is because 19/20ths of that air would now be cold and not needed to be blown down to your basement.
I just threw those numbers out. So think of it as a principle rather than a respresentative model.
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