How are the two players connected to the receiver? You are using a SONY BD player and an OPPO DVD player. Is either one of these connected via RCA (analog)?
I would unplug both players and try another source, for example a cable box. Make sure to unplug both players first and cycle the power to the receiver.
You said your receiver went into protection mode, this usually indicates a problem on the output (speaker) which you have swapped so probably not the cause. A speaker cable can also be the problem but again you swapped. The only other reason I can think may be the cause is a bad analog input say the DVD player over RCA. If the DVD (if it is connected via RCA) player output circuit is damaged, there could be a DC voltage present which quite likely cause an issue with the receiver.
