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 ?
You definitely need a sub for HT. I suggest you talk to your Triad dealer and I think you should probably double your budget. As a point of reference, take a look at Hsu Research and SVS.
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.
Nothing wrong with Velodyne at all. Lots of choices. Lots of price points.