The differences in sound between classic and D series that I can speak to mostly come down to the tweeter. D series uses the LPZ tweeter that is purely an in-house 100% Spendor design that gives them a modern, lively sound. Great sparkle up top, tons of detail, neutral not bight, but a modern sound. Not forward like Paradigm Persona or Bowers 80xd3 series or even Focal. The tweeters on all of those are much more forward than the LPZ of Spendor D series, just to keep things in perspective. But LPZ still has all the detail, and I love that about them. Note they do take hundreds of hours to break in and can sound bright during break in so be aware of that.
The tweeters in classic Spendor are typically third party soft domes that Spendor specs, and are great in their own right. More laid back but for some that is better. Since you are also interested in Harbeths perhaps you may be one. For my tastes Harbeth house sound a bit too laid back but that is just me (to be fair very little experience here, just heard couple times at dealer don’t even recall model - so I can’t offer any opinion of model you’re considering). To each their own. I have never heard the large classic Spendors, only the bookshelves (plus all of the A line). Obviously some of the larger classics have bigger cabinets and that will affect bass. Again, D series uses the most advanced porting system in comparison to A line or classic, which they talk about the ports being inspired by F1 race cars (seems a bit hyperbole but whatever).
I love the D’s. Some who love the historical Spendor sound prefer the classic line and consider the D line too much of a departure from the tried and true. For me the D is in the perfect sweet spot. All the beautiful midrange Spendor is acclaimed for but now with the top end to match