Scott E.:

Just so that I have a fuller understanding, what would happen to the bass sound with an 8D in a 20ftx20ft room or larger?  Would there not be enough volume to fill the room?  Or is it an air displacement issue?

What are people's thoughts on a 10D in a 13ft x 13ft room?  Is that too much?  I only ask because I may be going from a smaller room (13x13) to a larger room (20ish x 20 ish) in a few years.

In a 20'X20' room there's likely to be dead spots in certain areas due to the 8D not providing enough output to evenly fill the room. 

The nice thing about a sub is that it has its own volume control which allows you to prevent it from overwhelming your other speakers.  So even though it would likely be overkill in a small room like that, it won't sound unbalanced.