After many months of doing similar research, I came to the conclusion that Rhythmik is the best bang for your buck sub. Servo controlled. Super low response into twenties or even the teens depending on room configuration. I have two F12s in my studio and two LVX12s in my living room. The F12s are in a 16.5 x 13 x 10 foot ceilings space. The LVX12s are in the living room which is about 1000 Square feet as it connects to the kitchen and dining area.
I have not felt the need to upgrade or add additional subs in either location. They are both set to a reasonable gain and the LVX12s will shake my entire house at probably 60% of their potential output. Also, they have sounded more "musical" than the RELs I've encountered at the local audio shop. Which I assume has to do with their super low distortion, ability to handle sub 30 hz content with aplomb, the transient response, and the servo controlled mechanism doing it's duties to control clipping etc.
+1 for rhythmik here - they're like half or less the price of RELs for the same output and distortion from what I see on the charts... but I get that RELs have a reputation and have gorgeous enclosures