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thanks for the tips guys, I've done a bit more testing and this is what
I've found:

I tried running the connection to the amp to each of the subs in the
box (the box has connectors for each subwoofer individually) - I did
it one at a time to make sure the problem happened with each speaker
being plugged in alone. It did.

If I plug both subs in together (a set of cables running to each
speaker) - it produces the same problem.

When I got the box I noticed the positive jack on one speaker, and the
negative jack on another speaker were "unscrewed" - making it look
like the last person to use the box had run one wire to each speaker
and they were bridged internally. I gave this a try and had no luck -
no sound was produced at all, not even the thumping.

I'll double check all my grounds but I don't think that's a problem
(unless it's a problem inside the amp itself) because when I hook my
old amp back up and then plug in the new 2-speaker box, I have no
problems and it sounds ok playing music.