Just updating this thread because I had a chance to tape again over the weekend. This time I taped a rock band in a small local bar/club. The SPL in the room were very loud, and I was about 15 feet from the left stack (which also happened to be a newly upgraded PA). The result this time was excellent. I guess the motion effects I got on the last recording were, as Chris noted, due to the low volume in the room and the fact that I was pretty far back.
FWIW, I had the mics mounted in the same positions on the front pockets of my shirt. I tried mounting them on my shirt collar, but they're too heavy and they ended up pointing downward toward the floor. Despite the same mounting position, I did not notice the treble rolloff I experienced last time.
In fact, the only problem I had was not with the mics, but with the Sony PCM-M10. Unfortunately, the automatic input level control was accidentally switched on for a few minutes during the first song. I keep the unit in my pocket and it must have slid over while I was checking the input levels. Luckily, I caught it right away. I guess it's time to tape over those damn slider buttons on the M10.