crysis,
You often get these Logitech anti-fanboys that categorically denounce Logitech. The bass is a bit too much for *some people but you can turn it down. Since using my Z-2300 set I get frustrated listening to speakers with nonexistent subs because it just sounds empty. As far as computer speakers go, Logitech is completely dominant; whether that is due to quality or advertising I can't say. I haven't had a high-end Creative set, so I can't make that comparison. Any home theater speakers will generally be more expensive and less convenient. You can't compare Logitech computer speakers to bookshelf speakers - you didn't, directly - because that's in a completely different league. Short answer: if you're looking for computer speakers, get Logitech...
OP,
The Z-4 speakers are much better quality speakers than the X-530 and will also have much more substantial bass. I wouldn't go for the G51...
That music is best in stereo isn't entirely true; if you have the Z-5500 (and other speakers) then you can use digital input and then some Dolby wizardy takes place and you're music is 5.1-ified.
renvilo,
If you decide to sell your Z-5500 I'm first in line.