I recently upgraded a 7 year old 325w SVS sub to a new 750w SVS sub. I had moved to a home with a living area of over 7k cu ft.
The new sub puts out more power and to a lower frequency. Makes a noticeable improvement in this size room for movies. Music is not that much different since the older sub covered those freqs present in most music tracks. I think the extra power (from 325w to 750w) is most useful on the lowest frequencies present in movie tracks.
In your size room 300w might be enough depending on how loud you like to play your movies. Axiom sub's seem expensive to me but they have great policies on trade-in, warrantee, and service.
Good luck with whatever you choose.
