~$6K mini-monitor recommendations?

I own the Harbeth M30.1 and the Harbeth P3ESR. I think for your application the P3ESR would be a better fit. I have owned many of this type of speaker, Spendor, Kef, etc. and the P3ESR is my favorite of the lot. The KEF LS50 is not a BBC type speaker.
https://www.bing.com/videos/search?q=harbeth+p3esr+review&view=detail&mid=5D7EB69CCEBBAAE793...

If these monitors are near-field - on your desk - and given your budget, I would look at the line of professional studio monitors by Focal, Dynaudio, and others. 

What do you expect to get that the Kef LS50 can't deliver? I think you will just be throwing money away.

I think the same thing. I am going to both upgrade my electronics to use with my LS50's and also buy a bigger "bookshelf" speaker to keep in the same office as the LS50's,  the Yamaha NS5000. The Yamaha is much better than the LS50's ($15k)  and also has a almost full range sound. Crazy as this sounds I plan on placing the LS50's on Isoacoustic stands and place them on top of the NS5000 when in use, otherwise on a side bookshelf.

I am keeping the LS50's because it is an incredible monitor and I will use it until it falls apart. I do know that better electronics brings out the best in the KEF's.

BTW - If you are set on up grading and like the LS50 sound the KEF Reference 1 is a great choice. 

Never heard them, but always heard good things about the Genelec 8300 series if true near field monitors are your goal.

I second the harbeth p3esr I have the 40th anniversary and I use a sub but for near field they have a surprisingly good bass response on them so a sub shouldn't be needed. Plenty of reviews most recently from zero fidelity.