Bass may be omnidirectional, but rooms also have nodes. One trick you can use, if you've got a long enough sub cable, is to place the sub where you will be sitting (push the chair back a bit first, not on it!) and then play a test tone through it. Walk around ten edge of you room and note how the sound changes, it will sound quieter in some places and boomier in others. Now if you swap the sub for a place where it sounded boomy, it will sound boomy from your listening position, and if you place the sub in a place where it sounded quiet, its effect will be reduced. Use this to guide where to put your sub. Avoid putting it at the points where it sounds most boomy, unless you're into r&b.