@bdp24, whether or not a sound source behaves as a point source or a line source depends on its physical dimensions in comparison to the wavelengths produced. The Rhythmik/GR Research dipole sub is far too small to approximate a line source at subwoofer frequencies.
" The dipole null to either side of planars/line sources/dipole subs eliminates sidewall-to-sidewall modes..."
Not entirely, but it does take the side-to-side modes longer to develop. Remember the front wave and back wave are each spreading out omnidirectionally, cancelling where and when they combine out-of-phase, but not cancelling elsewhere and elsewhen. You can think of a dipole as two monopoles back-to-back, in opposite polarity, with a wrap-around path length separating them. After a sufficient number of bounces the frontwave and backwave have energized all of the room modes, but their energy is also dissipating as this is happening, so a dipole does have somewhat smoother in-room bass than a monopole.
But two intelligently-positioned monopoles approximate the in-room smoothness of a single dipole, and impact is preserved. This is why I use four monopole subs in my system, which is designed to blend well with two dipole main speakers.
"... the sound of the room itself being pressurized is conspicuously absent. Employing four subs instead of two does not provide that benefit, does it?"
I’m not convinced that a net unpressurised room is an advantage at low frequencies. My experience has been that good, solid low-end impact is facilitated by pressurization.
That being said, it is quite possible to configure four subs to not provide net pressurization at low frequencies. Just reverse the polarity of two of them. I have done this and imo it does some things well but impact is noticeably softened, so it is not something I recommend.
On the other hand, I generally find reversing the polarity of ONE of the four subs to be beneficial. Or as an alternative, the phase of two of the subs can be set 90 degrees apart from the other two. These techniques improve the in-room bass smoothness in the modal region while preventing excessive bass energy down below the modal region.
I owned a set of those Gradient subs when I had Quads and agree with your assessment.
As I said before, I have NOTHING against the Rhythmik/GR Research subs. I’ve chosen to do something different, which arguably also works well.
Duke