subs and small room

The first thing you need to determine is how low your sub is going to operate. Room modes occur higher 80-200 (mid-bass) region in smaller rooms. Check the internet on how to determine where room modes occur based on the distance between two walls. So it will be your main speakers that will excite room modes. The sub if it operates below 40Hz with a XO of 24dB/octave won't really cause a problem but may even help smoothen out nodes that may occur with proper placement and being a bit frugal with the sub's volume control.

Tobias,
Its not frequency doubling that occur with room modes but amplitude doubling or cancellation i.e peaks and dips.

Duke,
Multiple subs would only work in a large room. In a small room this would only create nodes (peaks/dips) at mid-bass and higher frequencies. So the result without any other form of control e.g EQ as suggested would be mid-bass prominence.

Samuellaudio,
Can I suggest you plot your room (Rives CD2 plus RS SPL meter) to know where you stand currently and then do it again with a sub i.e a loaner. You will then know for sure how much the sub affects the room.