I use one of these and it works great and is pretty inexpensive. Depends on how large your sub is.
[url]http://www.amazon.com/gp/product/B001140OZ0/ref=ox_sc_sfl_title_5[/url]
[url]http://www.amazon.com/gp/product/B001140OZ0/ref=ox_sc_sfl_title_5[/url]