I started trying to get estimates today, and with one company I contacted for an estimate on a Santa Fe Compact dehumidifier and an Aprilaire 600 humidifier, the owner told me that it sounds like the problem we have is not going to be fixed by a humidifier and dehumidifier. He said that we could very well end up spending money on the dehumidifier and humidifier, and then having them run up the electric bill, and still not getting to our desired humidity levels because our air leakage is high (it was 3667 at the first home energy audit, then insulation and air sealing brought it down to 3150 or so). He said that we should be trying to get the air tightness down to 2500 CFM50, and that he's done that for people, but it would cost a whole heckuva lot of money, a lot more than getting a dehumidifier and a humidifier, and actually even more than we spent on getting a totally new furnace and AC system. He said that we should also be taking a look at the duct system, which the home energy audit showed to be not performing efficiently. A ComfortMaxx Quick Check Customer Report of the cooling system a year ago showed that the duct system operating efficiency was 50%. He said that the amount of reduction in CFM50 is not enough, you can go down 500 just by insulating the attic hatch. He said it shouldn't be the case that the house quickly (in the space of hours) goes back up to 65% humidity after a normal cooling load brings it down to 50%, that means that we need more air sealing.
So now I'm stuck between a rock and a hard place. If he's right, a new humidifier and dehumidifier wouldn't solve the issue, the only thing that would solve it is spending much, much more money on air sealing and increasing duct efficiency. And I haven't got a big enough budget for that, sadly.