bleedlikeme, people here at Hydrogen Audio generally hold that the best-quality mp3 is the one that sounds closest to the original cd. The word "transparent" is used to describe an mp3 that doesn't sound any different than the original cd from which it was encoded. What you should do is use EAC to rip a song to uncompressed wav file, and then compare that to the one that you downloaded. If the downloaded song is much louder, then someone probably increased its volume before encoding, or possibly increased the volume of the mp3 file after encoding.
Songs that are louder usually seem to sound "better" to our ears, because we can hear details more clearly. But louder doesn't mean that the mp3 is actually better-quality - as AtaqueEG says, if you turn up the volume on the mp3 that you encoded from the cd using EAC and LAME, it will sound just as good (and quite possibly better) than the mp3 that you downloaded.
Interestingly, a common trick of some encoders (past and possibly present versions of Windows Media) is to boost the volume of the encoded track slightly, thereby making it "seem better" to a quick listen, without actually increasing the quality.
A danger of mp3's being too loud is clipping - if the volume is boosted a lot, then the loud sounds will start to hit the maximum "ceiling" of allowable digital sound, which can hurt the crispness of sound quality.
One tool that will be useful to you here is
mp3gain. It can be used to raise and lower the volume of mp3 files, but its main purpose is to standardize the volumes of mp3 songs, so taht you don't have different albums and songs that are much louder or softer than each other. Most albums taht are mastered at loud volumes will "clip" when encoded to mp3, and so mp3gain can be used to lower the volume.
Mp3gain also tells you what the maximum no-clip gain. I've seen some songs that I've downloaded (including the songs that I got off Radiohead's "The Bends" before I bought the album) had a maximum noclip gain of -7.5 dB - which means that the volume had been raised at least 6.0 dB after encoding, by someone who didn't know what they were doing or by someone who thought that louder is always better. perhaps the files that you downloaded are similar to these? (I know you said 75% max volume, but you might have meant something else by this.)