I'm guessing that you think HDTV compressed to a 3-to1 ratio looks better, too. It's the same idea as compressing music.
In your opinion, the mp3 only sound better because the high frequencies and low frequencies are rolled off to decrease file size. The rolling off of the upper range has the effect of "darkening" the sound you're listening to. It also adds artifacting into the sound. Instruments in the upper and lower echelons won't be accurately represented starting at the file.
No, each piece of equipment has different software and hardware inside. The file type is the starting point to creating a consistently correct sound. The software will decode the file type as it is programmed to. Some software does a better job than others.
Rather large generalization. Noise isolating headphones would be a much better solution to the ambient noise issue. You're just creating self-induced hearing loss by raising the volume of your headphones. Over time, you'll have to continually raise the volume of the player due to the loss of sensitivity you've incurred by such a self destructive habit.
Turning on and off the eq is a way to change the "tone" of the music. I'm assuming that you're more interested in being able to adjust settings at mulitple Hz levels as opposed to just doing a generic on or off.
Some light reading on mp3 compression and formats:
Site 1
Site 2
Site 3
Great quote from the 3rd link:
"You can get used to awful," says record producer Phil Ramone. "You can appreciate nothing. We've done it with fast food."
Happy reading,
-pat