I only listened to the WS99's briefly, but my initial impression s out of the box, are that they are more balanced than the M100's. The highs extend further and the mids are more forward. The M100's has more bass slam and goes deeper while the WS99's bass sounds tighter. The M100's also have a wider soundstage and a fuller sound. The WS99's haven't been properly burn in, nor did I do any extensive listening, so take these comments with a grain if salt, but so far I'm impressed.
Pretty much my impression as well. Both are very good for the reasons you mentioned- wish the WS99 had the M-100 wider soundstage/fuller sound and the M-100 to have more forward mids of the WS99...now THAT would be something