Hi, Sten. Because all three methods would employ the same dBNeutralizer material that Big Fat Dots are made of to accomplish the isolation and decoupling, all three should give roughly equivalent results. You could try whichever configuration you choose and most likely get satisfactory results that meet and exceed your expectations. As to whether one configuration works better than the others with your particular system and environment, you would have to audition each in order to determine that--not necessarily at once, but over time.
With most subwoofers on a bare floor, Four Big Fat Dots directly between the bottom of the cabinet and floor gives ideal results, no real need for spikes. (On a carpeted floor, you would use Giant Fat Gliders instead of Big Fat Dots.) Down-firing subs often require a minimum floor clearance, however, so using spikes can be helpful to achieve the necessary floor clearance.
Herbie's Audio Lab