Jay, You need to get yourself one of these https://www.parts-express.com/omnimic-v2-and-dats-v3-dayton-audio-speaker-measurement-bundle--390-809
Once you use one of these I think you will reject the word "flat" from your audio vocabulary.
In free air your dipole woofers radiate in a dumbbell pattern but in a room? All bets are off. Bass is omni directional and the wavelengths are frequently longer than the room. The interaction with room boundaries is complex. With a sealed woofer you are lucky to be +- 3 dB at the listening position. With a dipole woofer you are lucky to be +- 6 dD. Move around the room and the bass will go +- 10 dB in places. Just measure at the listening position then 6 feet in front and 6 feet behind then compare the graphs.
I use 2 measurement systems, one is an earthworks m23 which is phase perfect out to 23kh along with HOLM Impulse. It's nice because it's very precise and back when I was designing crossovers for my 3 different iterations of active 3 way speakers it was really useful. Now it's mostly used for bass measuring and EQ. But my other tool is a real time FFT set to 2 second delay and no smoothing from Digital Studio Six. I like this one because even though it's less precise than the Earthworks setup, it lets me be mobile. So I can walk around my entire listening room and see immediately, in real time, exactly how my speakers and room are interacting.
I've used sealed speakers and sealed subs in the past. Even when using multiple locations for bass generation, the room is always problematic. I have heard rooms that sound great with sealed subs, in fact a good friend of mine has a dedicated room that uses 5 Rhythmik servo subs and it sounds phenomenal. So you are right that sealed subs in a swarm can sound great. When we visited Duke's room at RMAF (Audiokinesis), they always sounded great with their swarm setup. So you are right, a swarm is a good approach.
However it's not the only approach. Self-powered OB Servo Subs will get you similar (or better) results in a real-world room. Because they are OB, you get les room ringing (bloat). Because they are servo controlled, you keep the "punch you in the chest" dynamics and power that normally only sealed systems get you.
And, if the only OB bass you've ever heard is a 15 inch pro driver mounted on a flat baffle, well I agree, that setup is quite underwhelming. But servo controlled OB bass like we are talking about here is on a WHOLE NOTHER level. If you haven't heard it yourself, then frankly you don't know what the hell you are talking about.