Room Acoustics and Speaker interaction.

I would like to have a secondary system in my small den which is only 11.5ft by 11.5ft with only a 7.5ft ceiling. Very small and very square...yikes!

So, because the room is so small, the speakers need to be close to the wall that resides behind them.

So this got me thinking about speaker types: sealed/front ported/rear ported, etc as I want to avoid a booming bass. But then I was thinking.....is it really the ports that are problematic or is it just certain low frequencies that are reacting to the room modes? Any thoughts?

How does one determine what frequencies to watch out for in your particular room?