I would say that buying a sub is almost mandatory in your scenario unless you get a really hefty set of tower speakers. The Focal towers would not really do well here because they are not effective at deep bass.
If you bought a different set of towers and center, you could likely still use the 906 for surrounds. I have found that the front 3 speakers need to be exactly the same sonic signature (i.e. same make/model). However, the surrounds don't necessarily have to match 100% as long as they are close.
Even though you "could" use the 906 for front left/right with a sub, I would still go for something different. Either get Focal towers or a different brand bookshelf that does bass/midbass really well. The Focal just don't have bass/midbass strength. The Focal tower speakers do "okay".
Even though you will have a sub that is likely crossed over at around 80hz, there is still a LOT of bass/midbass coming out of your left/right speakers between the 70hz and 150hz area.
If you bought a different set of towers and center, you could likely still use the 906 for surrounds. I have found that the front 3 speakers need to be exactly the same sonic signature (i.e. same make/model). However, the surrounds don't necessarily have to match 100% as long as they are close.
Even though you "could" use the 906 for front left/right with a sub, I would still go for something different. Either get Focal towers or a different brand bookshelf that does bass/midbass really well. The Focal just don't have bass/midbass strength. The Focal tower speakers do "okay".
Even though you will have a sub that is likely crossed over at around 80hz, there is still a LOT of bass/midbass coming out of your left/right speakers between the 70hz and 150hz area.