What is better 1 bigger sub or 2 small ones

Agree with the, "it depends", viewpoints. However, do not forget that subwoofers are speakers too. So do not percieve or treat them with any less consideration than your main speakers.

Multiple subs are harder to set up properly

I also disagree with this statement. I agree that it is possibly more difficult. But not necessarily. Once again, it depends. For example, if the room is such that placement of a single sub disallows it to ever be properly set up, that is, no matter where you place it, you have noticeable issues (i.e., frequency suck-outs and/or boominess), then a second or multiple subs might be used to eliminate those issues which a single sub cannot. In this case it is then easier with multiple subs to "properly" set up.

According to the book, "Get Better Sound", 2 are required for accurate imaging, and all the old saws about how bass is non-directional are beside the point.

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.

Its complicated, as all of life is. I have two, which I started with. Felt one would never tell me if two were not better. I am happy.