~$15K floorstanders: opinions sought

I think only one person mentioned Legacy Audio. Are the not well regarded around here? I thought the Focus xd’s sounded pretty good for the price

I am a big Spatial fan. Given that they are on your list, I would definitely try to demo a pair of X3s. I own Luminas and have heard the X2. The X2
 is something of a hybrid of the two. AMT driver like the X2 and powered subs like the Lumina (although not servo). As to your room, they need to be at least three feet off the front wall, I found five feet to be optimal. Other than that, the design is intended to reduce room interactions. 

"Krell high power sand amps . . . and JBL high-sensitivity speakers"
The preceding are at opposite ends of the spectrum.

My current two separate systems in different rooms of my home are
1. Large horns with SET valve/tube monoblock amps
2. Large Martin Logans with subs and high powered sand/silicon solid-state monoblock amps

Over the decades I’ve auditioned a lot of well-known amps. Voicing is very important and dependent on a lot of variables. At home I’ve swapped equipment around between systems. The horns are very revealing in a bad way. Many highly touted expensive amps are noisy as heck (no it’s NOT my system) at low levels and idle in a 106dB system. Hum, dirty, etc. If you listen to classical music, it will BUG the @$%%* out of you. Move the same amps to my low sensitivity 89dB system, quiet as a church mouse, sounding great, and awesome. The opposite is true, i.e. low powered amps in low-sensitivity systems sound pale, two dimensional, and quite honestly insipid.

Recommendations:
1. Although you CAN, I would not use high powered solid-state amps that have tons of NFB with high-sensitivity speakers. The sound is very forward, two dimensional, and in your your face. That gives the horns a way too loud harsh PA sound unfortunately. Then again, maybe that’s desired result?
2. First hear or audition the exact or similar combination of equipment with the understanding that it might not as good in your environment.
3. In the following order: start with your environment, then speakers (fit the speakers to the room, not the other way around. People buy a system because it’s rated well, then spend $$$$$$$$ trying to get it to work in the room), then amps, then preamps, etc.

Trust your ears but define the type of sound that you are pursuing. Note the room characteristics, the golden ratio, and challenges with acoustics. You will save yourself a lot time and money. There is nothing worse than spending $100k on equipment and tens of thousands of dollars on room treatment only to find out that it doesn’t sound good no matter what you try. Then you try to convince yourself that you've spend major bucks on the sound, so it sounds great, doesn't it? Been there a few times, and done that, and won't do that anymore.

Like me and my opposite ends of the spectrum systems, everyone should have more than one system, because one size does not fit all media sources, genre, etc.

Good luck

Of those you listed:

1. Spatial Audio M3 Sapphire or possibly the X5/X3

2. Magico S3
3. JBL 4700 - but, terrible resale value 50%, or less.

But... my 1st choice would always be that of many recording studios...
Legacy Focus SE/XD, or the ATC SCM50 or SCM100 (which are way too expensive).

Harbeth 40.2’s are also superb... but... require careful matching to the right amp (ie. Hegel H390 or H590).

If you have $15k to spend, you MUST audition the New Sonus Faber Olympic Nova III speakers. I auditioned them but not in my price range. I've owned the Venere 2.5's and now own the Sonetto V speakers. The new Olympica's are outstanding!