>>>> Other songs mastered on cheaper speakers (e.g. NS10 Yammies) sounded very good.>>>>>
So as part of the monitoring process, the product must be auditioned on a variety of speakers, icluding cheap ones. Listen on a walkman, a boombox, on you home stereo, in the car. It is often surprising that a mix that sound great in the studio does not work in such a real-world situation. You have to mix for all of these situations, because this is what your listeners will be using. Most big studios keep a small boombox and an automobile setup on hand for this purpose.
>>> I did not know these speakers have built-in enhancement circuits. Do you have an idea what types ? >>>>>>
The specs for these circuits are kept pretty secret. The Z5500's are labeled "THX Certified." This spec is acheived by adding circuitry that alters the sound to agree with the THX idea of what sounds good. These specs are usually licensed, and it could be that even Logitech does not know what the spec is. It may be that THX requires the inclusion of proprietary chip of unknown design. Creative, Dolby, DTS and the various game designers all have their own specs and the people who make this type of speaker work hard to make their equipment compatible with as many standards as possible.
Here is a link about THX, etc: http://timefordvd.com/tutorial/SurroundSound.shtml
And here is an outstanding site for people new to desktop audio: http://www.audiominds.com/