I owned the Sanders 10c and used them in a dedicated room for about a year. Amps were Sanders Magtech Monoblocs, the EQ/crossover was done with Behringer DCX2496 (Sanders later changed to the dBx Venue 360 for the 10e). I also had the Sanders Preamp and Sanders cabling. Later I removed the Behringer crossover/EQ and used my Maui Modded TacT RCS 2.2XP to do all EQ and crossover functions.
Yes, these speakers beam highs--meaning at least the top two octaves--more like a laser beam than a flashlight. Move your head more than a couple of inches left or right and the highs coming directly from the speaker disappear and the sound is what I call "muffled." I think the reason some people comment on this and others don't is that some people don't care or mind what their speakers sound like from outside the sweetspot, much less outside the listening room. Other folks find this muffled sound outside the sweetspot to be annoying and thus complain about beaming, even while acknowledging (as I do) that this high-frequency beaming is exactly why the Sanders have such truly exceptional stereo imaging and staging. That beaming reduces the degree to which the speakers reflect sound off your listening room surfaces, or at least it CAN do so.
Now, this "muffling" outside the sweetspot can be substantially reduced by reducing or eliminating absorptive wall treatment. But then the strong suit of the speakers, their incredible imaging/staging in the sweetspot is significantly compromised from the ping-pong bouncing of highs all around your room surfaces. I've never liked the sound of an undamped room with any speakers, but the Sanders made the lack of damping even more obnoxious. The highs don't "light up" the room with ambience from their reflections as most other speakers do. The reflections are still audibly discrete reflections--ping-pong-like, as I said. Thus, I damped the room with lots of Sonex and lived with muffled sound outside the sweet spot.
Now, let me say right away that perhaps I just never got the hang of how to properly treat the room for the Sanders. At AXPONA demos of the Sanders speakers over the years, I've never heard either extremely muffled sound outside the sweetspot or ping-pong bouncing of reflections off the walls. Sanders demos at AXPONA have always had absolutely top-tier sound quality, as good as any other room at the show. Sanders sometimes set up the speakers on a room diagonal, but not always. He often used heavy drapes behind the speakers and some sort of curtain or screening between the speakers.
Two other issues I had: first, the bass was not strong enough to balance with the panel at the factory-recommended setting of the equalizer/crossover. When I goosed it via the equalizer to measurably match the panel level down to 20 Hz, the woofer box/driver made buzzing/rattling noises in protest at levels above moderate with material with heavy bass. Maybe I shouldn't have spiked the speakers to the concrete floor beneath the carpet, or maybe I was just expecting too much bass extension. But I was used to speakers that could go flat to 20 Hz in this room and the 10c's could not manage that at any level above quite moderate. In fairness, I have heard Sanders speakers at AXPONA several times over the years and the demos there were always top shelf with absolutely no bass problems even at quite high levels. And I know Sanders has made improvements to the woofer in the 10e.
Second, the level of quiescent hiss (hiss from the speaker panels with no music playing) was quite high, clearly audible even outside the listening room. This was entirely eliminated when I swapped out the Behringer crossover/EQ box for the TacT, so somehow the Behringer unit was the cause of that problem. I consulted with Sanders about this problem and he was very cooperative but unable to hear any such problem in his installation. As I said, the 10e now uses a dBx crossover/EQ box and I've never heard any such hiss from Sanders AXPONA demos.
Even with the TacT, I eventually got tired of the lack of usable bass extension and the muffled quality of the sound outside the sweetspot.