Not sure you are still interested in this topic. I didn't have anything to say about it before, but I do now.
Sounds like your model B is a fine instrument. I’m sure you are maintaining it properly, but remember that over time the sound will change from the hammers becoming harder, which will produce a brighter sound. Of course, that all depends on how much it’s played.
I happen to be reading a lot about Steinways right now and from what I’ve read, a new Steinway will need some extra attention when you first get it home to bring it to your taste in voicing. But a used one that is already settled in will need attention at some point too (as any piano does).
And is your model B piano American or German? Supposedly the American Steinway has a softer hammer than the Hamburg hammer and a new one may have a slightly muffled treble sound because the hammers are not hard enough. This can be changed though by a technician adding a hardening solution to the felt. A new Hamburg piano on the other hand has a harder hammer and technicians spend extra time pricking the felt in order to soften it. These voicing issues are something to think about when deciding upon a new or used piano and also American versus Hamburg.
One book I’m reading now is “The Piano Book” by Larry Fine. He states several times that ultimately, you should purchase the biggest piano you can afford because you’ll get a better sound. So I think you should definitely look at the model D since you can afford one and have the space for it. As far as quality in new versus used - since Steinways are mostly handmade, then the quality is probably the same. Although I’m sure that advances in technology makes some things easier today. Somewhere in that book it says that it used to be the Hamburg piano was made with better quality control than the American, but today the two are more equal.
So…you have a tough decision to make (unless you've already made it). But obviously in a used model D, it will depend on a lot of things. A brand new one, though, is a whole different ballgame. Good luck and let us know if you went Steinway model D shopping or if you're keeping your model B.