For both recordings, there was a significant difference between ratings of the 64 kbps sampling rate and the 128 kbps sampling rate, but no difference between ratings of the 128 and 256 kbps sampling rate. It’s looking like the 256 kbps MP3s offer no advantage over the much smaller 128 kbps MP3s.
Out of nearly 700 participants, 33 were able to detect a difference in audio quality between both sampling rates. I bet those 33 will be identified as being avid KUR readers! :)