Can I check the amp's negative terminals for continuity to ground with a meter? Or check for continuity between the negative terminals?I was about to post the following response when I saw your subsequent post, which makes this no longer relevant. But I'm submitting it anyway, as an fyi:
I'd feel very confident that doing either of those things won't hurt anything, but I'm hesitant to say that with 100.00% confidence.
What you could do instead, as a temporary connection arrangement to confirm that signals are present on the amp's - output terminals, would be to connect the + inputs of the sub to the - terminals of the amp (for each channel), and one of the - inputs of the sub to the ground screw on the back of the amp. If you hear sound from the sub with those connections, it would confirm that signals are present on the amp's - output terminals.
Best regards,
-- Al