In the past I have read about people having two or three subs and have always wondered why they find that necessary. With my system I use a Infinity SWS 212 sub-woofer. My receiver is set flat on the sub output and the actual sub is set to around three on the volume. When playing movies using these setting this sub will shake the whole house. My house is brick and about 2600 SF and I just don't understand why anyone would need multiple subs unless the unit you are using just can't do the job. So back to my original question.... Why do you think more than one sub is necessary (assuming the first one is a decent sub)? I have always wondered and just had to ask. Thanks, Jeff