Okay, I have a JBL Synthesis® set up and the K2 S9900s, but not in the same system. I have K2s as a stereo pair on the large, open main floor, supported by two Synthesis® S2S subs below 40 Hz.
My Synthesis® system in a dedicated room on the bottom floor is a One Array, so it has everything the K2 Synthesis® system has, except for the K2s. Instead it has the SAM1HF and SAM2LF modules. The SAM1HF units are like the horns on top of the 1400 Array with a Ti UHF and Aluminum HF driver, and the SAM2LF units are dual 8" anodized Aluminum mid bass drivers.
A K2 Synthesis® system would be a ball buster, but my room is too small at 15x17x8.5. The One Array system is so powerful that the consultant I hired (a former JBL Synthesis® employee who told me what to do; otherwise it was all DIY construction until he did the calibration) recommended it as being at the extreme upper end of what would be comfortable in that room. The K2s might be dangerous. (kidding, sort of)
As a stereo pair, the K2s and S2S subs have no problem filling our expansive main floor (about 10,000 cubic feet) and even our upper floor, and they sound particularly compelling in the "sweet spot" after a small amount of room treatment. As an HT, the One Array is shockingly good. Of course the room was built from scratch with secondary walls, insulation, acoustic panels, diffusers, bass traps, and so on, and, of course, got the full JBL calibration.
I would recommend sticking with JBL (Lexicon and BSS) electronics for two reasons. First, the system approach that JBL uses functions best with known electronics. The calibration software already knows the characteristics and capabilities of every unit, and once the software is told which main speakers, surrounds, pre/pro, SDEC units, and subs are in the system, it can calibrate specifically on the data it has in memory of the performance characteristic of every single component. Throw in a Denon, Marantz, McIntosh, whatever, and it's simply an unknown, random factor.
Second the SDP40 (rebadged Lexicon MC-12 with a few tweaks) isn't used for the EQ. The SDEC units are, and they're far more sophisticated than any Audyssey or Trinnov automatic or semiautomatic system you'll find in consumer gear. They're actually customized BSS Blu units with JBL firmware in them, and that's where all the work is done during the calibration. They're PEQ, crossover, microprocessor, multiple 24/96 DACs, and I/O units all in one. That's another reason why the calibration can be so precise. These units (and the software) know where a SAM1HF and SAM2LF unit should crossover and can set it. They also know the efficiency of each unit, FR of each driver, etc. They can assign the crossover point(s) for the subs (2 or 4), change phase, shave off unwanted peaks, etc., all under the guidance of a trained calibrator. You don't just put out several mics, push a button, and hope for the best.
As for the subs, I'll step forward and say that the S1S-EX subs definitely aren't one note wonders. Each is powered by a bridged S800 stereo amp, putting out 800W into 8 Ohms, and you'll feel that in every frequency from the crossover point down to the bottom (in my case 17 Hz) without a 1dB drop in output. (Even at 15 Hz, it's only -3 dB.) Your pant legs and your hair will move, if you have any hair or pants.
I have nothing against any of the other suggestions, and you'd be well-served to check them out with anyone who actually has them in daily use. I'm obviously biased, since I've voted with my dollars. The good news is I have not regretted the purchase a single day--that is not a single day after I finished the room (took four+ months), got the equipment out of the boxes and installed (another two weeks), and had the calibration completed (another week). Up 'til then I was doubting my sanity, but when I heard it as it was meant to be, I knew beyond all doubt that I had done the right thing.
And my girlfriend agreed!