i didn't mess with Winamp much when i used it, but right off the bat, if you're using a 5 speaker setup for 5.1 surround sound, it either isn't automatically configured for it, or... flat out doesn't support it. Windows Media Player always used every speaker for me. going by actual sound QUALITY.... i didn't notice any difference.
Winamp "can" produce better sound, but it totally depends on your setup. Winamp is capable of using waveout versus directsound. Some sound cards behave better with waveout. There is also a winamp plugin to allow sound to be passed via ASIO which is a low latency, pro recocording protocol; your sound card must support this. There is also a plugin which can bypass windows built-in mixer (kmixer) which can also result in better performance. This is especially important if you have a Creative card or AC97 card that requires that all signals be upsampled to 48khz for output.
It is important to remember that output quality can only be equal to the input quality. There is no way to magically make a 128kbps Xing mp3 sound better.
I thouroughly do not believe in using dsp plugins as these alter the original sound. Also, EQ should only be used to compensate for definciencies in the speakers or listening environment.
Also, stereo sources are not meant to be played back in 5.1 (6.1, etc). Many processes are used to create the other channels, some with better results than others.
If you have a creative card, you can look into the KX Project drivers, which can significantly help your sound quality out.
All this is just general. If you list your equipment, then I can try to give you a little more in depth info.
Welllllll~~~~~ my equipments are exactly the ones that'll deliver good sound, tweak or no tweak... But uhh, here it is anyway...
Creative labs SB Live 5.1 and Inspire 5100 speakers...
I was never into pc audio (or audio overall for that matter) until now... just upgraded for recent games. So even this setup sounds pretty decent for me compared to the crappy half working 2 piece speaker i had for years... yup, didn't even have a subwoofer.
If this can be improved by some tweaks, drivers, or programs... i'd like to know.
I have yet to be able to tell a difference between the two, to be perfectly honest. I don't have a high-end speaker system, nor do I have 5.1 so I don't think I'm going to be able to notice any of the subtle differences between the sound the different decoders produce. I base my player choice on functionality. Oddly enough, I actually use iTunes and I've been pretty happy with it so far.