Masilu,
I used to use small speakers in my listening room but in order to build up a satisfying sound pressure level I would have to play the system louder than what I might prefer for a given listening session. By adding a sub I could develop a pleasing sound pressure level while keeping the overall volume lower. I described it as the system having a fullness of sound at lower volume. If this is the issue you are trying to rectify then I'd suggest a sub. On the other hand if it is overall loudness in the mid and higher frequencies that is the issue then I would consider more power.
I used to use small speakers in my listening room but in order to build up a satisfying sound pressure level I would have to play the system louder than what I might prefer for a given listening session. By adding a sub I could develop a pleasing sound pressure level while keeping the overall volume lower. I described it as the system having a fullness of sound at lower volume. If this is the issue you are trying to rectify then I'd suggest a sub. On the other hand if it is overall loudness in the mid and higher frequencies that is the issue then I would consider more power.