I think that REL is not differentiating here between class D and TBL class D. In any case I don't see how not connecting the black wire at all should ever result in a well defined behavior.More strangely, according to the REL manual, when connecting the high level input to class "D" amplifiers, the black wire should not be connected. In my case, the subwoofer then produces a continuous, deafening rumbling sound. Only when I connect the black wire to the minus, the rumbling stops, but the bass is weak. Compared to the same settings (level, crossover), the bass on RCA is at least 5 times more noticeable.
I'm at loss what's going on here.