You can actually play mp3pro files in regular winamp, though I don't know if the quality suffers.
There is a specific plugin you can use, there's one on
http://www.mp3pro.com/ But it seems winamp Input filters can only distinguish based on file extension, so the plugin has to take over playing of all .mp3s. This is bad, because it has some issues, and isn't exactly rock solid (I've had it crash twice in 5 minutes while playing a streamed mp3). The default mp3 filter has many more options and seens to work better overall.