I've listened to them for a few weeks now and I swear they sound better now.
Right. I'm sure they do.
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?
No, and its not only speakers. Everything sounds better when its running a while. At least as far as I can tell. There's cycles everything goes through.
From new: expect massive rapid improvement the first few minutes to hours, followed by continual gradual improvement for anywhere from a few days to several months.
From on: expect noticeable rapid improvement for several minutes to a few hours as the component warms up after having been off a while.
From whatever: expect small slow gradual improvement from daytime into the late evening.
None of this is theoretical. Its all based on actual listening experience with a lot of gear over several decades. Exceptions are rare, to the point I can't think of any, but I leave open the possibility.
Its not stiffening up because it happens with things like cables with no moving parts, and even things like amps where the parts are fastened and cannot move even relative to each other. I warm up my system every night both by turning it on and by playing demagnetizing tracks but even so there is no way to warm up the cartridge other than by playing records. I've tried enough permutations to know the phono stage, motor, and even the cartridge each go through their own warmup cycles.
Congratulations, you are a listener. One would like to think all audiophiles are, but sadly this is not the case. So congratulations, and welcome to the club within the club.