Now, you have a couple of options here.
This is so you can add another device, like an equilizer, betwwen the processing section and the amplifier section.
What is a line level converter, and how do they help? If you are going to buy a new one, you can check it.
There are various ways you can make use of your existing connections but you must know how to connect a subwoofer to a receiver without the right way to avoid damage.