Your old receiver had a 100 Watt amp built in for the subwoofer. That is why it used speaker wire connections. The sub itself is passive (no built in amp), where most subs are active (powered by a built in amp). The new receiver does not have a dedicated amp section for the sub. Instead, it has a sub pre out that is to be connected to either a stand alone amp or an amp built into the sub. In the old receiver, all the decoding and bass redirect took place in the pre-amp stage (as does the new receiver) but it had an amp section to feed the signal to for the sub. The new one does not have this amp section built in.
Your options are to replace the sub with a powered one, buy an external amp to power your sub (manual shows as 100 Watts max, 8 Ohms), or go back to the old receiver.
You may be able to connect the new receiver's sub out to the old receiver's AUX in (Left or Right), then connect the sub to the old receiver as before. This may get complicated and it may put too much processing on the signal. But you can try it.
Something like THIS
will do as an external amp. There are many other options.
Also, read the label on the sub warning about connecting to any other amp.
What you are going through has been and will always be the problem with HTIB systems. People buy the HTIB as a system, then they want to replace components. It just doesn't work.