Your boiler and you baseboard heating has no adjustment for increase humidity. Whoever told you it is impossible to have low humidity with baseboard heating doesn't understand the science of relative humidity.

How old is the house, and how tight are the windows and exterior doors? I suspect cold dry air is entering into the house and this is the main reason your house has low humidity. You should look into stopping air infiltration as much as possible.

Humidifiers need to maintained and cleaned so it is always best to use them as a last resort. However if are doing research of models then the Aprilaire 865 is one to consider. It is a steam type and is designed to be used without a HVAC duct system.

Built in 1986 and you are probably right. The house is very drafty, cold air coming in through front closets and the windows are crap. I got estimates for new windows and the cost is ridiculous. The humidity is causing damage to new hard wood floors I put in. I need to do something to fix it fast.

A word of caution about the use of humidifiers in cold climates: the moisture introduced in the conditioned living space will create a driving force for moisture transfer to the (drier) exterior. Condensation in the wall cavities can result depending on how the home is insulated. If condensation occurs, it can result in growth of mold/mildew in the wall cavities. Proceed with caution.