I would say that it it were for a short period of time (a drop once in a while), it may be fine. They would not thrive at 73 but they will not necessarily become ill if everything else is good (water quality, food, no internal parasites). They will likely eat less at lower temps.
However, as a long term to permanent (late fall through early spring) type of situation, it will not be good for them.
I see you are in Michigan, which is substantially colder than where I am. Thus, I suspect the lowest temperature your basement may get may actually be lower than 73. Thus not good.
One possibility is to insolate the aquarium (s?) as much as possible, such as with pink foam panels on 3 sides and bottom, to minimize heat loss. You may be able to maintain the temperature somewhat higher. Another may be to heat the room, but that can get quite expensive. Humidity can be problematic depending in rom characteristics. Good luck!
