Jadis J-2

There's a pair for $10k. Get a pair of Spendor D9 instead.

If you want them, then go get them.  Why care what anybody else thinks?  You don't know what they sound like and manufacturer support will be iffy.  So what!  This hobby is irrational -- be irrational.

I can see the visual appeal, gorgeous, and technical appeal, i.e. 93 sensitivity, easy impedance range to drive, but no way would I try/buy them,

Down Firing Woofers, NAY.

I have some experience with down firing woofers, I recommend you go for front firing ones, and tilt them back a few degrees to change the angle of dispersion of the drivers relative to floor and ceiling (eventually rear wall) and aim your tweeters at seated ear level. Toe them in to produce a specific ’width’ of L/C/R imaging, and change the angle of dispersion from side walls. Stability with ’possible’ movement (not spikes).

my 15" woofers began life down firing, up on 8" high bronze legs, in a Fisher President II Console made in 1958.

My 1st custom separate enclosures, to free them for wider and toed-in placement, were a pair of exact boxes, on 8" legs, simply moved the front panel with horns and crossover, placed the woofer over a bottom hole, and moved the back panel with it’s pair of L-37 L-Pads to the new boxes. (see L-Pads below).

Next, I made my current enclosures, taller to face the woofers forward, (also increased internal volume of the box for a bit of bass extension), tilted back a few degrees as I mentioned, and an optional rear port designed together with Electro-Voice engineers who were still in NYC in those days.

Rear Ports, Passive Radiators ... Yay, Nay.

1. YAY! Prior space, no wall behind.

I left the ports open, in my exuberant youth ’got more bass’, whoopee. Tweeter and midrange horns Imaging awesome, low bass ’fuller’, how about that! Low bass imaging: I thought (had read/been told) all bass was mono, didn’t expect any, just listened for the new-found extension.

2. NAY! This space (most), closer to corners, a rear wall. (see System Photos here). Happily Electro-Voice engineers designed it to be open or blocked.

Bass Imaging is Real, Go for it.

I blocked the rear ports. Wow, now I discovered low bass from those 15" woofers could have imaging, distinct locations of bass players became apparent. I learned here, it may not be the fundamental, i.e. 30 hz, but the overtones, 60, 120, 240 give the imaging, your mind figures it out.

IF you have front firing woofers (or a stereo pair of front firing subs) with no ’omni-mess’ from down/side/rear ports or radiators.

Swarms of Subs must be awesome bass extension, but I suspect they are properly used with low crossover points and with Mains that already produce enough front firing low bass and those front firing fundamentals create bass imaging with a huge ’floor’ of bass added. Not for me, I don’t have the space or desire, and I don’t like organ music, I’ve heard the cannons, cool, done with that.

Precise Balance is a Big Part of Success

Any 2 channel imaging is Pseudo Imaging, there is no sound waves from center, which is why I can have excellent imaging with a dining table in front of me. Which is why I want remote control of balance, instant, speck by speck, from my listening position, via my Chase RLC-1 remote line controller. Remote volume also. A small balance tweak can make a surprisingly large improvement to imaging, ’everything’ opens up when it’s right. That is what is ’right’ about many people’s favorite recordings, the engineers ’got it right’.

L-Pads

Speakers are NOT designed for your Space (or your Ears, or Preferences).

ALL Vintage Electro-Voice, and many other makers provided L-Pads. The crossovers and ’starting’ frequency response were designed with Mid-Level Attenuation. 1st one Mono speaker had these, later 2 stereo speakers had these, for a while a separate Center speaker and center amp or center l/r mix output. A Klipsch (which one, I forget) started out as a center speaker, they later changed it for stereo pair use.

Distance from corners effects bass volume relative to the mids and highs. Rooms are average, dead, or alive to varying degrees. L-Pads, starting in mid attenuation were designed to individually boost or cut the volume of the mid and high drivers relative to the woofer’s volume (in your space), and relative to each other. ’Presence’ for Mids; ’Brilliance’ for Tweeters.

L-Pads I discovered here, are different than Pots, L-Pads maintain the same impedance shown to the crossover no matter what. Pots alter the impedance to the crossover.

Very careful adjustment, takes me weeks, are needed to refine the levels, but when done, superb! I would not want to live without L-Pads.

1st, system balance must be refined (prior, these mid-settings, or another pair of smaller sealed speakers).

Next each speaker must be adjusted from the Woofer’s bass output when in your preferred location.

Critically, you must get the L/R speakers frequency response matched, so frequencies don’t wander off center, i.e. Piano notes, i.e Singer’s voice, unless they are walking around the stage or studio, you want stability for all frequencies, that takes time I assure you, you can make a mess, speaker builders have chosen not to allow this, so you now need to find speakers that ’fit’ your space, and off to the room treatment solutions.

To solve L/R matching, still providing adjustment for unknown spaces live or dead, the most advanced design Electro-Voice produced was in their Model Six. They made a network of 5 specific resistor based frequency curves, and the specs published those specific response in a single graph.

https://products.electrovoice.com/binary/E-V%20Six%20EDS.pdf

Maybe I will have a pair of resistor networks made someday, try them, my speakers are similar to the six, like a little brother, only 15" woofers, only two horns.

My Oh My, I do blather on. Whenever I do, I always remember this:

TMI (too much info). "other than that, you confuse me with someone who gives a ...."

2. Is it completely insane to buy decades-old speakers,

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Technolgy has moved onwards and upwards.
For this  budget I'd suggest you consider  Voxativ and AER single band drivers.  Both labs have build on  the old Fostex/Lowther ideas and have made stunning success.
AS I always say, 
**New is not alays better**
 That can be read  in 2 ways.
New, as in xover designs(excluding some of Troels Gravesen's designs, as very competitive with the new AER/Voxativ) , vs the old technology from the 1920's. 
(= new not better vs the old)
The 2nd meaning is  ,,,New wide bands are in fact superior to the old wide band full range. 
In general the new wide band outshoots most every single xover speaker past 50 years in the midrange.
But in general the Troels top line competes with AER/Voxativ. 
At least this is how i add things up, Won'y know til I acutally hear a  Voxativ. 
Then i will know one way or the other about 
Troels vs new wide band.
Which will outshoot the other in the 500hz -4k hz.
My bet goes wide bands out performing the Seas. 
As I already have the Seas Thor, which is nearly iddentical to Troels top speaker from Seas , his CNO 25 Mark2.