Ditto most of what’s being said here. EQ isn’t some magic bullet to make a crappily compressed song sound better, or to make a crappy set of amp/speakers sound like a high end system. In any case if it is needed, it should be applied on your receiver, and always the same for every audio source. All those EQ presets you usually find in software players -like Winamp- or cheap boomboxes are plain garbage.
Equalization is needed at the recording/mixing studio to deal with room acoustics, that affect some frequencies, or because of asthetical reasons, like the lead singer not being drowned by the sound of the guitars. Anyway, the resulting mix should be presented “as is” to your sound system, and ideally your system should have a flat response throughout. If you room has some acoustical problem -there’s no ideal listening room- then it would require some precise tuning done by someone who knows a bit about sound calibration, most usually a trained technician. That kind of proper use of an equalizer has very little to do with applying bizarre EQ presets like winamp etc… and it won’t be applied at the source.
If you apply arbitrary EQ to your sound files, just for the sake of it, you’re basically saying that you know better than the audio engineers that recorded and mixed the tracks in the first place, and I’m afraid 99.99% of the time you’ll be wrong. It’s pretty much like the guy who goes to a backalley tuning shop to get his 10 yo car customized; 99.99% of the time you’ll end up with an even crappier looking car.
Oh and btw, if you use the digital output the blame on the poor audio quality is entirely on your audio system. My music (encoded with flac, no crappy lossy encodes) sounds incredibly well on my Onkyo receiver with Kef speakers.