Peter, although speakers, especially small ones, will typically exhibit distortion before amplifiers which have adequate capacity(which yours appears to have)will audibly distort, this certainly shouldn't appear at a "medium" volume. If you connected the M3s in exactly the same way that you had the T2s connected, and didn't get similar distortion, it would indicate a defect in the T2s. Do both(all?)T2s exhibit the problem if you try them one at a time?
Incidentally, the 150 watt maximum power rating doesn't mean that the very loud sound level with that input would be delivered with low distortion, but that the speakers wouldn't suffer permanent damage with that input. At the medium level you mention(were you able to measure it with an SPL meter?), the T2s might be using 1-2 watts.
