hey
Nimitz87 - well I dont have the logitech Z550 but i do have a 7.1 surround setup using Creative Gigaworks S750, the sub is brilliant but due to my loction (smack bang in the middle of a block of flats i ahve too really limit the sound that is directed to it and redirect it through the main speakers, which still gives a nice full sound, good definition between the various speakers, it would also depend on the soundcard you have ,I would assume, for configuration of distance to seating to give optiumum surround effect.
http://uk.insight.com/apps/productpr...03OJ4&src=FRO1
this will give you all the specs you need for this range of speakers, i would highly reccommend as i ahvent seen it dropp of the top end list of speasers from alienware, i bought my rig a good year and a half ago also so im surprised at that
hope i helped :-)