Beginning to develop my plan...

[reverse]

My ultimate goal is for clean and effortless bass in addition to gaining a little bit of extra floor space instead of having a sea of black speakers along the wall (living room, not a dedicated HT room). I could probably suffice with just two 18s but like having dual subs to help with room modes so am looking at going with 2 manifolds along the front wall mounted in the attic.

General Info:

• Room is 19' x 15.5' x 10' and is also somewhat open to an additional 3k cu ft.
  
• Ceiling joist spacing is 21.5"
  
• Mains are Energy Connoisseur C-7 (three 5.5" woofers - freq. response: 34 - 23 kHz)
  
• Surrounds are Energy Connoisseur C-1 (one 5.5" woofer - freq. response 53-23Khz)
  
• Ceiling manifolds (if dual) will be mounted almost directly above mains. If single manifold, it will be between mains but slightly left of center.
  
• Powered by the infamous, but ugly as sin (I'm a designer), EP4000 and tamed by the BFD.
  

Questions:

• I posted the freq. response for my speakers above due to them being 5.5" and a post Thomas made regarding radiating resistance. I'm sure 4x 18s are more than enough and would outpower my mains but just want to make sure this isn't an issue if I'm not looking to turn it up beyond what my speakers can handle as they will be crossed at 80hz anyway.
  
• Will two 18s be enough for the space or should I stick with the dual setup (four 18s) even though I will more than likely never utilize the power they would offer. I'm not looking to listen as loud as I can tolerate, just need it to fill the space and dig deep while being effortless.
  
• Even though my ceiling joists are rather wide, can I get by with only making the opening 22" x 12" if my manifold was built to be 22x22x12 (interior dimensions) for just two 18s or will that not be big enough?
  
• In the event of the manifolds being placed above the mains, will this cause the sound to smear, if so I will need to go with a single manifold slightly off center and between the mains.
  

[ThomasW]

Can't see any plot but that's not important...

My comment about 5" drivers relates to single units not multiples.

Yes no problem doing a pair of 2-18"s firing through a 22"X12" opening

Manifolds above the mains won't cause smearing, actually that one of the more optimal locations

[reverse]

Thanks Thomas, I just want to make sure I won't be creating a post in the IB Mistakes and errors section.