Vikvilkhu, it sounds like you're thinking of high efficiency main speakers that don't go down very deep plus an equalized sub.
For an optimal small-room system my own thinking is along similar lines, though you might consider using two or more small subs. The reason is NOT to get louder or deeper bass - the reason is to get smoother bass. You see, in small rooms a severe peak-and-dip pattern is inevitable no matter where you place the sub, and equalization alone is not a complete solution. By using two or more small subs placed asymmetrically, each will produce a different peak-and-dip pattern at the listening position. The sum of these dissimilar peak-and-dip patterns will be smoother than what you can get with a single sub, and this will be true throughout the room. Combined with EQ this is a very effective approach.
For an optimal small-room system my own thinking is along similar lines, though you might consider using two or more small subs. The reason is NOT to get louder or deeper bass - the reason is to get smoother bass. You see, in small rooms a severe peak-and-dip pattern is inevitable no matter where you place the sub, and equalization alone is not a complete solution. By using two or more small subs placed asymmetrically, each will produce a different peak-and-dip pattern at the listening position. The sum of these dissimilar peak-and-dip patterns will be smoother than what you can get with a single sub, and this will be true throughout the room. Combined with EQ this is a very effective approach.