Hello, today, i have made a little trial with a well known "killer sample" .
Using the default command line "-b 128 -m j" i get the sample "killersample128CBR" and with the command line "--alt-preset cbr 128" i get the sample "killersamplealt128cbr".
I find the first result much better (or least terrible ) than the second.
You can check here:
http://jjlapin.free.fr/codecs

:

Funny... I did the same test few month ago. Even gogo 3.1 is better to my ears than --alt-preset on this sample. But on regular music, I often hear better results with --alt-preset than with -b settings (or Gogo ).

You can not conclued anything on one sample (this one is really strange, and non-representative).