I was going to write a long reply, but see that Martykl has already written more or less what I would have said. So I'll tick an agreement with him, and summarize in different words:
Multiple subs placed using a moderate amount \of trial and error will nearly always provide a smoother, less erratic bass than would a single one. The biggest problem with a small room or a large room isn't its size but just that it is a box with incredibly irregular low frequency response from all the multiple standing waves. Multiple subs let you choose placements that compensate some of the unavoidable holes and peaks that any of the subs singly will give you.
Multiple subs placed using a moderate amount \of trial and error will nearly always provide a smoother, less erratic bass than would a single one. The biggest problem with a small room or a large room isn't its size but just that it is a box with incredibly irregular low frequency response from all the multiple standing waves. Multiple subs let you choose placements that compensate some of the unavoidable holes and peaks that any of the subs singly will give you.