Thanks for publicizing this excellent info.
I recall seeing schematics on the web for two or three older (non-REF) ARC power amps which showed the 4 ohm tap as being connected to the amp's circuit ground, with all of the other taps (including the 0 ohm/common terminal) outputting signal voltages relative to circuit ground. And, as might be expected, circuit ground being connected to chassis ground/AC safety ground through a low value resistor.
So I suspect that a similar approach may be used in the REF amps, which would explain why connecting the black (ground) wire from the sub to the 4 ohm terminal makes sense.
Also, I would think that if connecting the positive wires from the main speakers to the 8 ohm taps, rather than the 4 ohm taps, results in better sonics from the main speakers there would be no reason not to do that, while leaving the REL connections as stated. And I would think that connecting the REL's red and yellow wires to the 16 ohm taps rather than the 8 ohm taps should also work fine, with the black wire continuing to be connected to the 4 ohm tap of one of the amps.