Subs volume getting lower?

I'm noticing over time That my subs are getting lower and lower. At first I thought I was just getting used to them but its getting to the point where is noticable to anyone who gets in my car. None of the settings were changed. Same radio and wiring, the only thing that's changed really is the temperature with me being in the chicago area its getting pretty cold. Could I have knocked something loose or maybe the speaker wires are going bad?