A good subwoofer setting does not calls attention to itself (overpowering) but you feel missing the lowest note when you tun it off. Probably you should adjust the subwoofer level with music that has continuous and decent amount of bass instead of using U571. Or since you have SPL meter, you may use test tones and adjust the level such that your main speakers yield the same SPL as your subwoofer. As for subwoofer placement, if you don't mind crawling around, place your subwoofer at your listening position. Next, crawl around the room while playing music with decent bass. The location where you find the bass most satisfying is the best location for the sub. Good luck.