Hi
Depending on which speakers you are using (Bookshelf, Satellite, Floor Standing), check what is the lowest frequency emitted by them...then go for a sub which can perform lower than that....no point going for a sub which can only go 5hz to 10 hz lower than what is already achieved by the speakers....
We humans can hear between 20hz to 20khz......so anything closer to lowest to 20hz is ultimate bliss of performance.....
If you are more into music....a sub which can go as low as 30hz or 35hz is good enough....if into movies....lower the better.....lots of subs are available which can perform at 24hz....
Review and decide......subs performance is not about the power but it matters about design, placement, ported / sealed, size of woofer / driver etc.