There's definitely less sibilance without the grills... you really need to try it..
Well, I quickly opened the grills and tried to listen for a little while without them. Here are my thoughts:
1) The entire upper midrange/treble region is noticeably elevated. On the Yulong A28, it's mildly noticeable, but on my amp, it's almost a night and day difference for me.
2) After listening, I have some ringing despite having a lower volume than I'm used to: I nominally listen at around 70-75dB for live recordings, but it's 65dB this time, measured with my phone. Probably too quick and dirty, but still something. I dropped the volume immediately when I heard the elevated treble, but that wasn't enough to reduce the ringing. 75dB would have been painful.
3) Soundstage is indeed more open and wide.
4) Cymbals, guitar strings, etc... become sharper for me personally, and they are really hard for me to listen to.
5) Sibilance is galore. Some of my recordings white-out at higher frequencies altogether.
6) Midrange loses the body and becomes somewhat ethereal, floaty,... not necessarily bad, but it loses the physicality of the sound for me.
7) Bass texture takes the hardest hit. I think Bass is somewhat more punchy, but I feel it was due to a lack of low bass rumble and impact, which the stock grill allowed.
At this point, I think for me personally, I might just try to put another piece of microfiber fabric behind the stock fabric to diffuse the sound even more. Opening the grill makes the sound much too bright.
I'm starting to think the reduced sibilance with this mod is due to the whole region being elevated, so sibilance wasn't as noticeable. But for me, personally, I'd need a source that's much darker than the Gamma2 DAC and my amp (my amp is darker than the A28) in order to enjoy this.
So... I guess I'm in the "LCD-2 with more open soundstage" or "cleaner HD600" camp rather than the "HD800/HE-6 camp" after all. And this little experiment also has had me sort of wondering if I should take a different approach than Jerg's if I ever get to modding my HE-560...