Z906 is underpowered crap with shitloads of connectivity options half of us will never use.
I second this very strongly.
I recently bought the Logitech Z906 and they have very good reviews for gaming. My usage is somewhat similar to your.
Gaming + movies 70% STB + Music 30% The Z5500 can never die but its officially discontinued by Logitech. Both z906 and Z5500 have 2yrs warranty and you will find their service centers in almost every district city. Edifer... Very few service centers. But thats not the issue. S550 do not have any digital (optical/coaxial) inputs.
Where as the Z906 has 2optical, 1 coaxial. Total 6 inputs. Wins handover to S550. I stay in a rented place so thumpy sound and loud 'Noise' is also not an option for me. I have experienced that Z906 perform very well at low sounds also. IMO try the Z906. I was skeptical when I read Z906 online reviews. It was mixed bag. Specially owners of Z5500 did not like it as compared to their mighty Z5500but still I auditioned it at a friend's place for few days. He had both the Zs. And decided to go for Z906.