Speakers for leading edge, transients, speed and big sound

My first speakers in 1973 were Klipsch LaScalas. While they were amazing for their time I have moved way beyond them. My favorite speakers now are Legacy Focus SEs. Bill Duddleston at Legacy is an incredible speaker designer.  I highly recommend you check out Legacy. They have some amazing stuff. Bill is someone who understands speaker designer. Before you buy check them out. I am an electrical engineer and this guy understands engineering. CHECK THEM OUT!!!!

Given your desire for speakers that capture lifelike leading edge transients, natural speed and dynamics, and a big, full-scale soundstage I would second those who would point you to a quality pair of electrostatic speakers. I have a pair of Sound Lab G7-5Cs in a room roughly the size of yours and they do all that (and much more). Not sure where you are located or what amplification you are using but feel free to reach out if you would like to explore this path further. Sound Lab make the finest stats available and that’s why I’m a dealer.

I would definitely look for a pair of Acapella Audio Arts speakers. 

One of the best horn speakers available. 

They use a plasma driver for the tweeter, which could be the best tweeter available. There is nothing with better transient response than a tweeter with ZERO MASS!  

Amazing attack and decay.

At your budget, you would be able to get a great pair used. 

I have a question about the JBL K2 S9900.

TMR just listed a pair, and I was looking into it to see if it’s something I should be interested in.

According to their datasheet, bass extends to... 33 Hz @ -10 dB? Really? Are they some kind of Cerwin-Vega tribute? Lots of folks talk about them like they’re the second coming though, so what am I missing?

The AMT (Air Motion Transformer) and it's "derivatives" have excellent transient response.  It's a matter of whether the rest of the components can keep up.