Absolutely need some sort of high pass, without them damage to the subwoofer is sure to follow. Richard strongly recommends using the Model 5 filters, however you can use the X series filters (commonly used with 2Wq subwoofers), a home made substitute for the X series (what I use), or electronics with the high pass built in (I think Ayre offers that). In addition to damaging the subwoofer I think not using a filter would sound horrible, way too much low end.