So if I understand what your saying is to connect the Black to the chassis on the W4S or use the XLR inputs and connect the Black to the Right or Left RCA input on the W4S.Yes, those are the alternatives I am suggesting, but be sure to note that I referred to connecting the black wire to the ground sleeve of an RCA plug (that would be inserted into one of the amp's RCA input jacks), ***NOT*** to the center pin of the RCA plug.
Regarding XLR pins 2 and 3, even if those are inconsistent between the Oppo and the W4S everything would still work fine. The only consequence would be an inversion of signal polarity, also referred to as absolute phase, the audibility of which is controversial, at least on most recordings. And which could also be corrected for by reversing the + and - connections to the speakers, although that would also reverse the polarity for any other sources that may be in use.
Regards,
-- Al