basshead gives good advice, but I wanted to address your placement issues.
#1 RULE OF SUBWOOFERS: The most aesthetically pleasing location is almost never the most acoustically pleasing location.
Originally Posted by barhoram
I can get about the same response in the front right or left corner, but the room looks better with the current placement...so that's where it is at today.
have to decide if you want it to look good, or do you want it to sound good? Rarely do the 2 meet....however, in my room they actually did!
The first thing you need to do is find out if you actually even need more subwoofers....don't get me wrong, more subs is always
a good thing, but a lot of people throw money at subs before they have optimized what they already have.
- Have you done the sub crawl to find the best placement for your current sub?
- Have you measured (or are you willing
to measure) your response (REW, Omnimic, SPL meter + graph paper)?
- Have you tried other placements (the back corners look like great candidates)?
- What preamp and what correction software are you using? And are you sure you have optimized your calibration routine?
- Have you optimized the phase (on sub amp) and sub distance (in preamp) settings?
Concerning localization - where do you have your crossovers set (and are all speakers set to "small")?
Your room is only ~2500 cu. ft., there's no reason that DD15 shouldn't be performing in that small of a space. Let's see if we can't get it working before you start throwing money around.