I haven't spent much time with the grills on, so I can't answer your first question. The grills do seem to have an impact on off axis response (left to right), but i can't say for certain. I actually think my speakers look better with the grills on, but in my mind I believe the grills must have a negative impact on the sound, which it makes it difficult to relax and enjoy the music.
I had a temporary setup where I had a queen size matress on the floor in front of my couch. I actually really enjoyed listening to music from that location with my head propped up against the front of the couch. I was probably a couple feet below the level of the tweeter. The sound was still very good, but the highs weren't quite as present. On some music, I thought it actually sounded better, as it gave a more relaxed/less bright sound, but for "critical listening", I prefer to be at tweeter level.