upgrade path from my current speakers? (Dynaudio Special Forty)

Given all the speakers you have tried, and how much you like the Dynaudio's I would suggest speaking to a Dynaudio vendor and ask them which speakers up the line would carry forward and improve on the characteristics of the Special 40's. 

Others have mentioned the rest of your gear and I agree you might want to consider some upgrades. You could make some big improvements without spending a bundle.

As an aside, I am considering a speaker upgrade from my LS50's, and Dynaudio is on my list. Any comment on the difference between these two?

I agree with the sub suggestion.  Lots of used subs on the market.  Do 2 as they are easier to integrate. 

Re: Dynaudio; My at home experience was Contour 20 was a significant bass response upgrade from Special Forty.  Plus you don't lose the excellent natural vocal soundstage. 

Also add:  The resale market for your Special Forty's is very strong.  I'd reach out to some of the retail resale vendors as well as posting.  If you have original box and materials you may get near what you paid for them.

The Special 40 is unique in the lineup.  If you liked the HS you should enjoy the new Confidence floor standing models as they share the same tweeter.  I’m a huge fan of the Esotar3, it strikes the perfect balance of being detailed while never getting shrill or bright. The S40 uses a modified version of the older Esotar2 so perhaps the Contour i series would be a better fit. Either way I’d stick within the Dynaudio family if that’s the sound you prefer.  

Or do what I did and move the S40 to the bedroom and go with an entirely different type of floor stander for some variety. I went Klipsch Forte since nothing fills a big room like horns. If I had the space I would have went with La Scalas + subs.