Is "mega bass" preset in "Walkman" series is just an equalizer or does it makes use of some circuit to enhance sound while this preset is in use? I mean suppose if we take SE K550i which does not have "mega bass" preset, and set equalizers in same position as in mega bass preset, then will K550i produce same sound effect as mega bass preset??

Mega Bass is a preset. I don't believe you can simply adjust your equalizer and achieve the same level of bass. At least I haven't been able to on my w580i. You can get close but it's not quite the same.