There was less corruption/error detection in BASS 2.3, which meant that more unpleasant noises (eg. squeaks) could get through. You can hear an example of that at 53s in the 11759.MP3 file with BASS 2.3, which is removed by 2.4.
BASS doesn't know exactly where the errors are (eg. what frequencies), just that there is at least one of them in a frame/granule, so the only options available are to reject the entire frame/granule or let it (including the errors) through, ie. the errors can't be corrected. It may be that sometimes the errors are small enough to not really be audible (as at some points in your example files), but there are also times when seemingly small errors can result in unpleasant noises.
If you would like to be able to disable the error checking, here's an update to try...
www.un4seen.com/stuff/bass.dllIt adds the following BASS_SetConfig option...
#define BASS_CONFIG_MP3_ERRORS 35
Set that to 1 to ignore the errors. Note it won't disable all error checking, just this particular type of error (in the Huffman coding).