My room is similar in size to Athipaul's and my sub also only switches on when bass signal is present. Although with mine once it's on it rarely goes off while music is present; it has a ten minute minimum. I feel like I'm hearing good clean bass in this small, well-treated room.
It was all done through experimentation as I initially ran the system with zero treatment for a few days before purchasing the treatment material. The sound was all over the place and bass notes were boomy or nonexistent depending on where I stood/sat. Now the bass is clean and even anywhere in the room.
How low does my system go? Can't say exactly as I don't have any measuring device, but tracks I use when auditioning equipment for bass reproduction are working very well. I suppose you could go the scientific route; get out your calculator and measuring tape but your ears will make the final judgment.
I would think with a room your size, getting solid, even bass distribution would require even more work and a quality sub or two. Good luck.
