Nope...
With you being a musician you should be looking at studio speakers (monitors).. The Z906s are no better then the Z-5500's, they are infact cheaper made Z-5500's without the nice control pod with the LCD. Thumbs down to logitech on this so called update... But if for some reason you want to update the Z-5500's (say they blow up).., take a look at the Edifier S550 5.1 Speakers... There really is no other speakers that are 5.1 for pc's that are any good anymore apart from Z-5500's and the Edifier S550 5.1 Speakers.. Other option is to buy an AV amp and speakers.
Take a read of this..
MONITORS versus HI-FI SPEAKERS
http://www.soundonsound.com/sos/jun02/articles/monitors.asp
http://www.soundonsound.com/sos/jul02/articles/monitors2.asp