Hello haywood310,
I think going from one Hsu ULS-15 sub to two is a very significant improvement that begins to smooth out the perceived bass response peaks and dips in the room, increases maximum bass impact, reduces the bass production required from each sub, improves bass dynamics and even improves the soundstage imaging.
I believe going from two ULS-15s to three will be a further significant improvement in most, if not all, of the areas going from one to two provided. According to Earl Geddes, going from three to four subs will only provide a marginal improvement but, in my experience, utilizing four subs was the threshold to attaining near state of the art bass performance.
As for configuration, settings and positioning of all 3 subs, I'd like to know more about your system and room before offering suggestions. Here're some clarifying questions:
1. I understand this is for music only but do you plan on incorporating home theater in the near future?
2. What are your room dimensions: length, width and ceiling height?
3. Along which wall are your main speakers and equipment/rack-long wall or short?
4. Is the room carpeted and are there any irregularities (large openings, doors, odd sections or shapes)?
5. Are you wanting to optimize performance at a single listening position or do you want good performance over more than one seating position?
6. If you just want a single sweet spot, where exactly is this seat located within the room?
Thanks,
Tim