Testing AAC 128 vs. 256 kbps sound quality

In test of 128 vs 256 kbps AAC files (i.e., what iTunes 7 calls "high quality" vs. "higher quality"),none of the ten subjects was able to tell the difference. Slashdotted on 5/31/07, where there seems to be a consensus that on good speakers the 128 sounds "tinny" and the high frequencies are distorted.