It has to do with the center to center distance of the two woofers. This frequency who's wavelength corresponds to this distance is the point at which comb filtering will start occur. It's also affected by the crossover point that is used. Most MTM's have the woofers much further away than the A/V-1+'s do. And Danny is using a fairly low crossover point so the off axis response is much better than a standard MTM.
Also, the AV/C is simply designed this way so that you can put the port on the front if the speaker has to be placed inside a cabinet. That's why the tweeter is offset. The woofers aren't any closer together than in the standard A/V-1+. If the speaker doesn't have to be placed inside a cabinet, Danny much prefers the standard design with the port in the back even for center channel use.
BTW, my dad is using 3 identical A/V-1+'s across the front and they sound wonderful together!