OP, In my experience, yes. I had the same situation as you where the previous owner hadn't used them in a while. The bass was anemic when I first started playing them but, after 10-15 hours or so, they started to loosen up. It's not hard to imagine that rubber surrounds need to be played to stay flexible.
Do you ever have to re-break-in speakers?
Odd question but I recently purchased a barely used pair of speakers. I'm sure he listened to them long enough to break them in but then he moved to another model and they sat for quite a while before I purchased them. I've listened to them for a few weeks now and I swear they sound better now. Do new-ish speakers stiffen up again if left unused for a long period of time? Or are my ears just getting used to them?