Q: I'm wondering the pros, cons, and theoretical tradeoffs of flown subs as well as what's possible with BASSBOSS subs.
Boundaries and proximity are the relevant points here. Subs on the floor are near the boundary that is the floor. Outdoors, subs can be moved away from that boundary by flying them. The theoretical advantage of subs on the floor is that the boundary that is the floor prevents energy loss into that hemisphere of space. (Hence the term half-space.) The measurement of the output of a source that is taken in open space will measure 6dB higher if the source is against a boundary, ie the floor, than if the source is also in open space. If, on the other hand, the microphone is against a boundary and the source is in open space, the result is that the measurement is effectively in half-space. In other words, it will read 6dB higher than if neither the mic nor the source were against a boundary. Humans are hard to fly, so they are usually on the floor. At the operating frequencies/wavelengths of subwoofers, humans are effectively against that boundary, which puts us in half-space anyway. Whether a subwoofer is against the floor or flown, since the audience is usually on the floor, the difference to the audience is minimal.
What does make a difference to the perception is proximity. If someone is hearing subwoofers that are 8 feet away on the floor, the level at that distance will be higher than if the subwoofers were flown 30 feet away. To achieve the same level in the same place would require more SPL from the flown subwoofers, usually requiring more subwoofers.
Proximity also contributes to the myth of throw. If subwoofers are downstage center, the distance between the subwoofers and the listener in the front row is a very small compared to the distance from the subwoofers to a listener in the back row. (Perhaps a theoretical 8 feet to a theoretical 128 feet, so a difference of 120 feet.) If the subwoofers are flown, the front row listener may be 30 or more feet from the nearest subwoofer. The back row listener may be a few feet farther from the flown subs but the difference is far less. Let's say that the front-row listener were to experience 120dB at their location about 8 feet from the subs.
Introducing another boundary, such as an infinitely tall and wide wall, would decrease the range of space into which the sound radiates by half again, so 1/4 space. The measured/perceived SPL in that region would increase by 6dB. This does assume that the source is immediately adjacent to the wall and the floor, in other words with no distance for reflections to be generated. If the source is far enough from the boundary for reflections to be generated, the increase is inconsistent at best, and the reflections can be destructive.
Add a second wall, (third boundary) and the remaining slice of space is 1/8 of a sphere, aka 1/8th space. If the source is at the intersection of these 3 boundaries, the result is another increase of 6dB in measured/perceived SPL. If the source isn't immediately adjacent to these boundaries, the resulting reflections can cause destructive interference, resulting in little or no perceived increase in SPL.
A ceiling is also a boundary, so subs flown near, or not near enough, to a ceiling can seem worse than subs flown outdoors.
The reflection from a boundary presents as a polarity-inverted mirror-source. The frequencies at which destructive interference occurs between direct and reflected sound vary with the distance between the source and the boundary. The distance from the source to the boundary influences the time at which the returning signal arrives at a given point, thus whether it is in-phase or out-of-phase with the direct signal. Some energy is lost over the distance traveled to the boundary and the distance returning to the point of reference. The amount of energy lost over that distance, combined with the rigidity of the boundary, influence the amplitude of the reflected energy. In places where the reflected energy is out-of-phase with the direct energy, the level is attenuated. It's also possible for the reflected energy to be in-phase with the direct energy, however it will always be delayed, so while the level may be increased at certain frequencies by the reflected energy, the quality and accuracy of the signal is still compromised.
Setting up cardioid subwoofer arrays can help to reduce the number of boundaries contributing reflections, or at least reduce the magnitude of the reflected energy, thus reducing their destructive effects.
A second source presents as a polarity-matched mirror source. Everything about arrival time and energy loss over distance is the same as with reflections. Where the distance between the listener and both sources is the same, all will sum positively. When the listener is farther from one source than the other, the difference in distance can be correlated to a wavelength at which destructive interference will occur. Thus the greater the difference in distance, the lower the frequency at which the destructive interference occurs.
Keeping the gaps between subwoofers (or arrays of subwoofers) to less than 1/2 wavelength of the highest frequency at which they're being used will help minimize the worst destructive effects. (That's about 7' for BASSBOSS subs.)
Now, to answer the really easy questions! The SSP118 and SSP218 are both equipped with fly-points. The VS21 is also equipped with fly-points and shares a dimension (36") with the MFLA, so they can be flown together using optional linking flying hardware.