I've read some technical explanation why AIMP was better than Winamp, but I don't really remember the details. There was also a survey that revealed that AIMP sounded better. However it might be very subjective.

Even if AIMP gives a better playback quality, you might not ever hear any difference in practice.

IMHO your hardware is much more important (audio card, speakers, headphones, even cables), as is the audio source (Audio CD, lossless or lossy codec).

And because I don't have any technical proves that playback quality of AIMP or Foobar is better than the one of WinAmp, I don't wanna suggest a better player based on some subjective opinions.

I guess that you should stick to the player which interface you like the most (may be other things like memory usage are also important for you). IMHO the features offered by AIMP are amazing. It is way better than Winamp. And it is absolutely free.