The problem is that your sub does not have a line level input, only speaker level.
The line voltage output from your processor can deliver no power only voltage.
The only way you can do this is to modify your sub amp.
The speaker terminals on the sub will have a divider network something like this.
Now the V outs will be connected to a mixer/buffer amp. so the channels can be mixed.
The purpose of this is to attenuate the speaker level to line level and prevent the speaker signals mixing and blowing up your receiver.
Now you need to remove Z1 and Z2. Now you need to find only one, and should use only one Vout.
You then need to carefully mount an RCA jack on the plate amp, and solder one V out connection to it.
This will convert your sub from a speaker level to a line level sub and it will work from and LFE out.
If you don't feel competent to do this, then you will have to place a power amp between the LFE out and your sub and continue using your speaker level inputs.
As mods go, this would be very easy if you have the skill set and tools.
