I haven't had to rename anything, I've had good success following the instructions..
Install.
Go to the original input decoder, and in it's settings you will find a box that lists which file types it will operate on. ( a list of three suffixes)
Delete these, leaving the box empty. This effectively disables the original Winamp mp3 decoder. No need to rename or remove anything.
Go to the MAD decoder, and check the box in it's settings to enable it. (as well as checking for 16 bit and etc)
That 's it. (may need to close and open Winamp)
To reverse, uncheck the MAD enable box, and go to where you were in the Winamp decoder's settings, there is a button marked "Default" which puts the three default filetypes back that you formerly erased.
Poof... you're back with the original decoder..
Easy!
I do this often... the only reason I don't use MAD consantly, is I don't get the file/artist info in the Winamp display or playlist, when I'm listening to streaming stations. I also don't get the auto updating website info that I normally get for some of these sites when I'm using the MAD decoder.
I still am not entirely sure this isn't me somehow, but I think it's a bug in MAD. Anyone have some insight?