DD's site says 35-40hz, but that sub is a newer version. What I've got is one generation before the current version I think. Not sure if they made any significant changes, but if they did the recommendations on their site might not be correct for this sub. Either way I'd like to tune it as low as I can reasonably get away with. Its going to be a daily setup, won't be going straight spl or anything like that. So I don't want it to a point it will destroy SQ altogether but I love hard hitting low end bass. I'm definitely open to suggestion.