I have not used the subwoofer out on the receiver before.

Maybe someone in here can help with that. Is it true that
the sub only kicks in at certain frequenmcies.

Also, my setup instructions from the receiver shows
an RCA connection out of the Yam and into some sort of
Yam sub. My question is: even if the connection to the
outboard amp is necessary for whatever add-ons Yamaha
sells, should I not hear something out of the speaks?

No info in the documentation. I'm going to have a friend
bring his meter over here and see if he can get a reading.
But nothing came through given I just did an alligator
clip connection on thin wire. Could be the cheap plug
I cut off was no good too. Lots of variables at that point.

I did the battery test on the speaks and they are both okay.

So maybe somebody can walk me through the troubleshooting.

The plug is located just below all the input jacks and
a tape out pair. The directions show a connection of
RCA at both ends to connect the add-on Yamaha sub.