Seriously, I really doubt that the satellites can compare to Klipsch ProMedia 5.1 Ultra satellites, let alone competing against Cambridge/Creative GigaWorks satellites. Also, there are bound to be some coloration from the satellites since there is a port behind each satellite.
As for the subwoofer, the sub driver might be larger but look at the size of the hole. It is huge like the Z-560/Z-680 which means that it probably is tuned at the high frequency like the previous generation subs. I don't think this new sub can take on the GigaWorks sub and obviously not going to overthrown king of all multimedia sub (PMU).
But all of this is just speculation since I haven't tested the Z-5500 in the same environment as my current and past speakers.
I am no fanboy of any certain brand. I will buy what sounds good to me and is a better deal. I have heard all these speakers in store demo, friends, etc... I actually had own the Z-560 (1st set - sold), Cambridge MegaWorks 510D (2nd set - still have), Klipsch ProMedia 2.1 THX (4th set - sold), and GigaWorks S700 (3rd set - still have). Overall, the Cambridge speakers sound the best to me not to mention the best built quality. Cambridge satellites are excellent in terms of neutrality and dynamic range. I do have to say that the Klipsch subs are amazing. Logitech speakers do have the volume but due to this they sacrafice some sound quality. But given their price they are well worth it.
I will say that these differences are minimal and barely noticible in a quiet environment. Only when you have bigger budget to spend or an audiophile wannabe would you care about these minor differences.