I have a 950 ft3 room with four 12" subs.
950 Cubic Feet, that doesn't tell us much, it would be better to have the actual dimensions of the room. However, if we find the Cube Root of 950, that is only 9.83 feet. That is a very small room. But if we assume 8ft ceilings, then the room is about 10.9 x 10.9 x 8ft
, still a pretty small room for all those Subs.
Next, where are the Subs place in the room, which direction are they facing, and what is driving them? By what is driving them
, I mean the signal source and the amps.
With FOUR 12" Subs now combined with TWO 18" Sub, I would speculate that you have a confused mess in the room. So, much bass bouncing around that it is cancelling itself out. Especially in a small room like that.
First, with out a buffer, you can only hang just so many inputs on a Line Level Ouput. A Line Level Output (Sub Out) does not have an infinite driving capacity. You are trying to drive Three Outputs if we assume you AV Receiver has TWO Sub-Outs. Otherwise you are trying to drive SIX inputs with the Sub outputs. Three is marginal but probably workable, but SIX might be a step too far.
Next as other have pointed out, this is not they way to determine increase in volume.
iirc, the um 18s modelled 121 db at 30hz. the 4 12 modelled 118 db at 30, so 121+118 =122.8
so I guess a 1.8 db increase?
I have speakers wtih 2x8" bass drivers each, when I turn on my second full range speakers with 1x12" each, I get close to a 6db increase in output as measured using Pink Noise. One 12" driver is about 30% larger than TWO 8" drivers. So when the second set of speakers come on-line, you get a bit more than double the air moving ability.
An 18" driver is approximately 49% larger than a 12" bass driver. So, using TWO 18" is like using THREE 12" bass drivers. I would guess you would have no trouble getting about a 9db increase in output.
Further, if you have Subs in Front that are Electrically in-phase with Subs at the Back, while electrically in-phase, they are mechanically OUT of Phase. That is likely to cause significant cancellation in the room.
While not a prefect graphic for this circumstance, it still illustrates the underlying principle -
I really don't think you need that many Subwoofers in that seeming tiny space. Likely you are pushing the air so hard and from so many directions that the air is in essence stagnant.
If you have about $100 to invest, then a good Calibration/Measurement Microphone
combined with Room EQ Wizard Software
which is free, should allow you to do a pretty decent analysis of what is going on in your room. Science can tell you so much more than your ears.
You will find a discussion of REW (Room EQ Wizard)
here with links, videos, and Microphone recommendations -
SPL meter recommendation
Frankly, I think 6 large Subwoofers in a room that is short of 11ft x11ft, is excess, and likely the source of your problem.
I would speculate, that if you insist on that many Subwoofers, you work very hard on proper placement and phase
of those Subwoofers to assure that they are working together rather than fighting with each other. And in that relatively tiny room, you will most certainly have to make some efforts to control the Room Acoustics.
Make of it what you will.