The issue with averaging is complex. As I worked out the math from Todds question I found that it wasn't so obvious. As a result, I have decided to keep the details of my analysis confidential for the time being. But here is the situation. No matter how you setup multiple subs, if they are spaced arround the room then they have to - in a statistical sense - lower BOTH the spatial and the spectral variance. This can be and has been shown by numerous qauthors, myself included. It turns out that there is some effect from pacement, but this is less than the effect from just the number. But in my experince placement is not usually completely optional. In other words the room will almost always dictate some placement restrictions. But as a rule of thumb, I find that one sub should be near or in a corner and the next one as far away as possible. The third one usually ends up wher it fits. OK, so far, number of subs most important, placement next in line and then finally you have the ability to set each sub up differently. This can have about the same level of effect as the location. How this last step is done is completely wide open and there are a myriad of opinions out there about it. I personally believe that in the end almost all of these techniques will boil down to just about the same thing. Todd uses a mass of measurement data and a mass of calculations to arrive at complex fitting functions which are then reduced to a "best fit" on the available parameters. I average at the get-go to reduce the data set and then "best-fit" to this reduced set. Will these two end up at the same point. Exactly probably not, but very close is very likely. So the question that is unanswered is what is the best way to setup multiple subs once you have the number and locations selected. There is no concrete answer to this at the moment.
If the mains have LF capability, then they should be considered as another source for the LF response - the more the better. SO leave them as "large" and don't HP them, is my recommendation. If you do this, then it shouls be obviuos that there is no advantage to a "ported" design and the mains should then be closed box.
The issue with my usage of ULF and BB subs is more complicated. Its based on "practical" not on some ideal. Ideal would be three subs that handled 20 - 200 Hz. But this is not "practical" so I split this decade into a pair closer to "octaves". The reason is that bandpass deisgns are very efficient but only over a small bandwidth. The one thing about room response at LF that I feel gets missed in these discussions is how the response varies arround the room at LF. At near DC there is NO variation of the response arround the room, it is everywhere the same. As the frequency goes up it gets greater and greater. But a sub operating from 25-50 Hz will se virtually NO spatial variation in either its response at the seating location or its sensitivity to location. Usuing more than one will thus only increase the output, but do little to nothing to smooth the response as there basically is nothing to smooth. generally ain this frequency range there will be a singular LF mode that is usually not very damped because damping is not very effective in this range. This is exactly where EQ works the best - the first few modes. My room has a mode at 40 Hz that no amount of sub control or placement can alleviate - only an EQ band can bring it under control. Thus (in my room) the single ULF sub is very efficient in the 25-50 Hz band and only one is necessary. Abouve about 50 Hz, more and more modes are evident and this is where the room response benifits from multiple subs - of which the mains are also sources in this region. Proper setup does make the region from about 50 Hz -150 Hz both spatially smooth and spectrally smooth. This is exactly the range covered by my BB bandpass subs. This approach WILL NOT work in all situation, but has worked in any room that I designed. At LF the room dominates the situation and every room is different. There is no one right answer in detail, but any succesful approach has to start with multiple subs placed arround the room (not stacked as this is really only a single sub). The size, bandwidth, power to these subs will be specific to the room.