I would definitely try the felt if I were having a similar problem to yours. Alan has recently mentioned that the M60s have great off-axis response, which means the tweeters send quite a bit of information out toward the side walls. This helps contribute to the perception of a wider soundstage. In your case, since your room is decidedly 'live' with all its hard surfaces, the off-axis response is probably what's causing the sibilance and ear fatigue.
In addition to trying the felt, or instead for that matter, I'd try to soften the room a bit, especially on the walls to either side of your listening position.
