I've been testing various subwoofer setups recently looking at alternatives to what I had been using, just to see what's possible. I keep coming back to a "tripod" setup, with two subwoofer locations in front, and one in back. I've been using two stacks of 2-subs each for the last couple years now, but this is the first attempt with using two stacks of 3-subs each in front. Each stack is two Dynamo 1100X and one 800X subs. The seventh sub is a 11 or 12 year old Rel 10" sub in "back". The sole purpose of the sub in back is to smooth the response, because without it the response is pretty bad.
I put the work "back" in quotes above, because this is also the first time I've successfully been able to place a sub directly behind my sofa. When the small Rel was in the back of the room I just wasn't able to get it to work in the frequency range needed to help the fronts as well as it does behind the sofa.
Just to be clear, for this example it doesn't matter that I'm stacking multiple subs in front, just consider them to be one sub each because it makes the conversation easier to just say I'm using 3 subwoofers.
So here is a plot showing what the front two sub locations look like, and then what the third location does to fill-in and make the response look so much better. And the Rel doesn't even "seem" like it's doing much, but it does!