What is better 1 bigger sub or 2 small ones

As with most things...it depends.

Good larger subs generally go lower with flatter frequency response. A single sub will sound more different in different parts of the room in many cases, so there is more of a sweet spot. Two subs enable more balanced response in more locations within the room concurrently.

Also the right sub depends on what speakers used with. To fill in the low end effectively with speakers that are more full range already, you will need at least one large sub.

So to get the best of everything in theory you want two larger subs. Good ones two.

If cost is a factor, then you have to determine what compromises you are OK with and decide accordingly.

Lloydc,

Theoretically that is true, but for the lowest audible frequencies that a sub would deliver in an already mostly full range system, how much it matters practically is debatable I think.

That using more subs enables smoother bass response throughout the room is a significant advantage nonetheless, if that matters. 4 is even better than 2 and 2 is better than one and the results in most cases should be clearly audible. Audiokinesis is the vendor in these parts who designs some of his speakers in a modular manner that allow for this by design up front.