This my theory when setting up subs. Use your ears to get it where you like it. If you need a reference level, the best sweep I've found is the subwoofer sweep on any THX certified disc. It sweeps down from 200hz to like 20hz (I think)and you can really hear if your subwoofer is set right by how clean the transfer is from your mains to your sub. You can also hear room nodes very nicely. Those are the parts where the volume goes up and down for no other apparent reason. Very educational for your system...