Lows: 0 ~ 250Hz
Mids: 250 ~ 4000Hz (majority of instruments and vocals lies here)
Highs: >4000Hz (some instruments, especially for the very higher notes and cymbals in a drumset etc, adds "sparkle" to music)
XB500 is bassier compared to XB700, the XB700 focuses on deep bass, XB500 got roughly the same deep bass response or tiny bit less at the deepest tones you can hear but packs a lot more midbass (typical pop song bass) which adds a sense of impact, feels like it hits physically harder. The also XB500 has less pronounced highs so it's smoother sounding. Since I don't know what you prefer it's impossible to tell. This hobby is all about taste, I can't objectively say to you headphone x is better than y because it could be the opposite for another person depending on what he/she prefers. You won't need any amp for these headphones but I do like using digiZoid ZO amp cuz it has 32 levels of bass adjustment and this "SmartVektor" tech adds a nice weight to the music, sounds more like listening to speakers than a headphone.
Equalizer is what you can adjust the frequency range with to make the headphones more balanced or more according to how you want it to sound like. The XB500 benefits by a boost to the mids and highs (500 Hz - 16 kHz) IMO.
Edited by RPGWiZaRD - 12/6/11 at 6:44am