Roundover above tweeter to minimize diffraction is theoretically real, but I doubt most folks could decern a difference in a real-world setting without test equipment. Within reason, I've come to pay more attention to what pleases my eye, but that's me.
As you're planning cabinet roundovers, pay some attention to where your woofer flange lands on cabinet. On X-LS Encore 5/8" would be close to interfering. 3/4" would have the flange sticking out over roundover I suspect.
Peter, I agree that the most radius I can do on an X-LS Encore is 5/8" because it uses the M-165 woofer with a 7-1/4" outer frame. The NX-3 towers use a pair of M-130 woofers that have 6-5/16" outer frames. On the 8" baffle, they leave room for a 3/4" roundover without interference. The NX-3 is a MTM (or WTW?) arrangement, so maybe the top roundover matters less than the side roundovers? There are bumps in the frequency response below 2kHz that may be related to diffraction when compared to the NX-Bravo and X-LS Encore.
Unfortunately (?) for me, I'm a member of a DIY audio group and have been shocked to hear, for example, the differences caused by changing only AC cables in a member's system. (Hopefully, I won't get flamed for saying that.)

While it dismayed other members, it made me laugh. Yes, things like these make audible differences in well-built systems but I get to decide if I want to make the investment to reach that far along the performance asymptote. Some members with good systems found that they started hearing things like air handlers in the studios if they dropped the noise floor far enough, and it started distracting them from the music. I plan to use the NX-3s as my speaker reference in hopes that bad things I hear are not because of the speakers.
I get what you're saying, and I get to decide if I want to pursue too many roundovers or make it simpler to build and view my cabinets.

Unless someone like Hobbs or Danny say that the upper roundover is important, I can spend more time and money trying to improve my listening room instead, which is in dire need of improvement!
