~$6K mini-monitor recommendations?

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

I owned the Raidho X-1s for a stretch. They are amazing when listened to at a distance, spaced 9’ apart and with 2-3 feet of clearance behind them. If setup isn’t perfect, they are not a special speaker. Just because a speaker is not rear ported does not mean distance to back wall is irrelevant.  

What is your projected distance behind the speakers? What about spacing between them?  

The Proac D2R is getting great reviews and is well within your budget. Might be worth a look.
Good luck

Spendor D1 (discontinued) if you can find them. Dealer I bought my Spendor D1’s from is licensed Spendor and Kef dealer and told me no comparison between D1’s and LS50’s. Of course the Spendors are double the msrp, so there’s that, but still well below your $6k budget. Hard part is finding them new.

I have Spendor D1 and SA1 (Spendor last two top-of-line mini monitors), and I find the D1’s to completely outperform the SA1’s in every way (but I am admittedly very partial to the LPZ in-house Spendor tweeter used in all D Series speakers as compared to the old 3rd party soft dome tweeter that the the Classics and A series use).

Spendor and Kef are both of the BBC speaker lineage, so if you are looking for that classic Brit sound, worth a look. Note that both D1 and SA1 are sealed (acoustic suspension), so I use a subwoofer with both as there is not a lot of deep base extension, as you’d expect. I have heard the LS50’s but only casually; have never given them a serious audition so I don’t have a personal opinion on strengths/differences between the two, but my dealer had both as demo’s for years and had compared them over many different amp/source combos.

What amp are you using and approx what are the distances the speakers will be from the desk/side walls/rear walls? Many of the recommendations you’re getting are totally wrong for nearfield placement close to boundaries!

At least start with models designed for nearfield. From the pro world: e.g. ATC SCM12 Pro (passive), Focal Shape 65 (active).

Harbeth P3ESR or LS3/5as should work, too, being sealed and very small.                                                                              

What about a Falcon Acoustics LS3/5A ??

I have no first hand experience, but they seem fun and suited to the task.

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.  

If you can find a pre-owned pair of Wilson Audio TuneTot they sound really good. 

Take a look at some of he Sonus Faber stand-mounted speakers. These are not usually abused so buying used makes sense. You can find Electa Amators (series I, 2, and even 3) used, for around your stated price range. High WAF is a bonus!

Man, those LS50’s are super sweet in a nearfield application... I know they lack dynamics and slam, in addition to being a little light in the bass.

What is your front end equipment driving them?

2 options not yet mentioned would be Fritz or Salk.  Salk’s wow 1 came to mind.  Spendor, ProAc,  Harbeth, ATC, and Dynaudio should be considerations too.

Audio Engine HD 6

Sorry, read ur post as $600. How about Esoteric MG-10’s?

So many good choices. 
I heard the new Elac Navis 
2 way active speakers
recently. At $2k they 
might leave you enough
to upgrade your source!
Maybe overkill but why not?

Have not heard the Boenicke W5 as desktop but 6moons tried them and said they work there too. They will of course image better with a normal setup. Great speakers in any case.

https://6moons.com/audioreviews/boenicke5/1.html

I would second the ProAc D2R.  I have a pair in my office system.  To me they are special.  Fantastic in near field.  Detailed but non-fatiguing.  

Give Focal Kanta 1s a listen...I'm a fan of the 2s, havent heard the 1s.

the persona b is on sale til the end of the day but I can’t remember if it’s rear or base ported. I listened to it and the kanta 1 back to back and there’s no comparison. I’m finding great value on this forum for dsp speaker systems that just a few years ago were cost prohibitive. 

Big +1 for the P3ESR, really nice near-field speaker.

While still rear-ported, the Audio Physic Step Plus is an amazing little speaker... huge sound stage :)

Legacy Calibre: I've not heard a monitor with this kind of dynamics.
The AMT tweeter is sweet and very extended, the B&C 7" midrange is
smooth and powerful. It's the best speaker I've ever owned-

pawsman

If you can find them, give the Ryan S610 a listen: They are truly stunning and well within the budget

Alta FRM2. There's a pair for sale on us audio mart. RAAL ribbon tweeters. You will not be disappointed.

ATC active would be my other recommendation, but understand those will not be very forgiving. Event Opal are also great actives. At $6k I'd go for actives tbh.

Danley Sound Labs Studio 1.  

https://www.danleysoundlabs.com/products/loud-speakers/studio/studio-1/

Tom Danley finally makes a speaker for the home.  $3200 for a pair.  $2280 if you want the sub.

Used Focal Sopra, excellent speaker and built solid,
the B&W 805-D a very well rounded monitor. Both solid choices a bit more lock on imaging then a ribbon ,ribbons may be a bit airy depending on equipment.