Midrange Better Than Harbeth or Spendor?

I think you are correct regarding my taste and the dip at 2-4Khz which is probably good in my small and somewhat reflective room.

If you're room is highly reflective then the Harbeth's may be the perfect "Goldilocks" speaker for you! Enjoy! Those radial drivers have a strong following and rightly so!

In their literature, Harbeth describes their speakers as "Fast, clear, precise, .... big rich sound with incredible ease and detail...dramatic improvement in clarity of sound".

I agree completely however I do not think they are telling the whole truth. Note the emphasis on "sound". Put the Harbeths next to Intuitive Design Summits or a single driver speaker and the problem is obvious.
Note these are only my humble opinions as to my preferences.

I can certainly understand in principal some of the adavantages of a single driver, but have never heard one myself. Which would you recommend that can match or surpass the clean and refined sound of the Harbeths or Spendors?

argument can be all day long for these matters, in the end, one has to go w/ what your ears tells you best. Free world, free country!

You could try www.carolinaaudio.com for the Jordan drivers although I have not heard them.
I don't think a single driver can usually match a multi-way in this respect since multi way puts the driver only in its most efficient frequency of operation. You can't have it all. At least from the very few single driver speakers that I have heard.
The best option I know of is running a 4" full-range and adding a supertweeter for over 10kHz to add some sparkle.