Ah yes, the infamous --alt-preset switches. For some reason unknown to me, the man behind r3mix (Roel) has been OOC at the site for a while, and hasn't been active in further development of the --r3mix switch. The --alt-preset switches are being developed mainly by a guy named Dibrom. This guy just wasn't happy with the --r3mix switch, so he's been using all of the latest features in LAME to develop the --AP switches.
For 256Kbps CBR, use this switch with LAME 3.90/3.91:
--alt-preset cbr 256
This will result in the absolute best quality you can get out of MP3s at 256Kbps bar none. You can get a complete list of --AP usage by opening a DOS command prompt window, and running LAME with the following switch:
--alt-preset help
I would personally recommend using
--alt-preset standard, because VBR is a lot more efficient with bits than CBR, resulting in higher quality MP3s that will usually average smaller than 256Kbps. The few reasons to use CBR would be compatibility with older players, or that you want to be able to predict the size of the files (as you never really know how large a VBR file is going to be). Otherwise, VBR is the way to go. Have fun
