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.