Jaz,

I can't comment about the 350 because I haven't heard it but I own the 600 and listened to the 400 with jakewash. I know that you don't need the 600 and it is highly unlikely that you need the 500. The 400 is indeed very smooth, will blend in very well and satisfy your needs. Other subs may give you the presence that you desire but they will likely compete with the rest of the music. The 400 will weave the bass into the rest of the music very seamlessly.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated