Lets start with placement. Use the AM Acoustics room mode simulator to try to find the best location for the speakers and sub. Try to keep your speakers and listening location out of the worst / lowest room modes.
Next, you should take Vandersteen's advice and use a high pass filter. Vandersteen sells them directly. Try to use a subwoofer for 10 Hz wide space is madness. Also keep in mind that the specs are garbage when the speaker is in the room, but many an audiophile who has attempted minimalist subwoofer use has become a convert to using a high pass filter, and one of the main reasons is that it makes integration a lot better and easier.