I guess it depends how loud you play music. In any event don't forget that the 400 belts out nearly the same SPLs as the 350v3 which is 2.4 x as large amd the size of a 500. For music it should work, though I'm a fan of the 400 pair as stereo subs if you want the best musical presentation.

If budget is less of a constraint I would get the pair and with the 600 you are well on the way to conquering your room. Otherwise adding the 400 for music and powering the 600 and sony for HT looks like a good plan with proper placement.

John