I have heard the JBL Everest DD66000 in private set ups with Emotiva, Accuphase, Jadis JA 120, a class D amp, and have also heard the K2s at many audio shows with Mark Levinson. I have heard the JBL 4520 in a private set up. Private set ups are those where the system is well-settled and I can use my own audition music.
Despite this exposure to JBL, I had never been a fan, except for this DIY JBL I accidentally stumbled on in Berlin. It is listed on audiomarkt by Musicplace, 10 minutes from Berlin airport, and retails for almost 44k Euro.
One of the best speakers I have heard so far, they use JBLs for midrange, and 18 inch Allnico drivers for bass. The bass was fired by 100w EAR amps, while the rest of the amplification was 8w Silvercore monoblocks. He used room correction as required to set them up, and can dial in crossovers to fit them i accordingly.
Nice room too, speakers placed at a distance of 4m with me at 6m. Just the most fluid midrange, excellent tuneful bass coming out of those woofers, oh, and the piano, I melted on the Bryan Janis piano version of Mussorgsky’s pictures at an exhibition. Jazz from Lalo Schifrin Blues in the Bassment, Symphonies from Scheherezade and Mussorgsky’s Pictures, were brilliant across violins, brass, and slams. Zeppelin, Natalie Dessay (Rigoletto Caro Nome), Bach’s violin concertos, were brilliant as well.
I have also met other designers like BD Horns (Bert) and listened to their designs. With all custom built horns from one men operations as above, there will be a preference to be geographically close to the designer, in case support is required. You do get the flexibility of finishing the custom design to your taste.