which subwoofer with Triad Bronze and AVR280?

I am trying to decide on a subwoofer to go with a system I am putting together in my living room (13'x15'8').
So far I have Triad Bronze Inwalls LCR plus Onwall Bronze Surrounds. There is an Arcam AVR280 on the way.

I am pretty clueless and would gladly accept some pointers as to what to choose. I don't even know whether to go for powered or not. In fact, some stuff I read on the web makes me unsure I need a sub at all. Budget is around 300.

I listen to classical, organ and (don't laugh) birds but if I'm honest I probably spend more time watching films.

Your help is appreciated.
Why not get the Bronze subwoofer that Triad sells ?
Post removed 
Thanks for the quick responses.
My Triad dealer decided that custom installations are more lucrative than competing with big-box stores - so he's gone. At the time when I bought the inwalls he suggested Velodyne since he claimed they are more value for money as compared to the rather nice Triad subs.
Post removed