Sure @martin-andersen, as with all dipoles, any of the GRR open baffle loudspeakers and subs can be placed as close as 3’ from the wall behind them (you call it the back wall, but if you’re facing it, it’s the front wall ;-) . 4’ feet is even better, 5’ better yet.
Sound travels at around 1’ per millisecond, and what we want to achieve is a 10 millisecond difference at the listening position between the direct sound from the front of the speakers/subs and that coming from the rear after being reflected off the "front" wall. A minimum of 10 ms is what is required for the sound of two acoustic events to appear to be separate from one another. 5’ spacing automatically creates that 10 ms delay (5’/ms from the rear of the speaker/sub to the wall, 5’/ms back to the speaker/sub), That 5 ms difference makes the rear wave sound like an acoustic event separate from that coming from the front of the loudspeaker, rather than part of the direct sound, a "smearing" of the latter. By the way, Danny Richie recommends the use of diffusion rather than absorption on the wall behind dipole loudspeakers. I concur.
As subs handle frequencies with very long wavelengths, the 10ms difference is not nearly as important for subs as it is for the shorter wavelengths/higher frequencies handled by the loudspeakers. Far more important is the phase relationship between the front and rear waves produced by each sub. The front and rear waves are in opposite polarity (180 degrees apart) relative to one another, and as the long wavelength bass frequencies "wrap around" the sub OB baffle, they meet on the two end sides of that baffle and cancelled out (one of the reasons dipole subs don’t energize as many room modes as do non-OB subs); + 5 added to - 5 = 0.
The rear wave also of course travels to the wall behind the sub, is reflected back towards the sub, and when that rear wave meets up with the front wave can create an either in-phase addition or an out-of-phase subtraction of lower frequencies. OB/dipole sub users need to experiment with sub placement to optimize the phase interaction between the front and rear waves. That might sound like a daunting task, but when you hear a good OB/dipole sub you understand why it’s worth it!
