Adding additional subwoofer

I would do a pair of 113's first. then if that is unsatisfactory do the pair of 110's. That will definitely solve the problem. You will have to keep the crossover point below 80 Hz. With two 113's placed inside the main speakers you can cross higher up which may or may not be beneficial depending on your main speakers. The other approach is to move the listening /viewing position back or forwards a bit till you get the bass response you like. One sub is never enough. It is just marketing to get you hooked.   

If you haven't already I'd suggest trying this first.

https://www.youtube.com/watch?v=AV3oLLMgS-M

Map the entire rooms high energy bass mode locations if you decide to add more units.

Check with JL Audio to see which 110's can be slaved using the F subs A.R.O..

For economical long cable runs check with Blue Jeans or Monoprice Cable. Have fun.

What frequency is this void? If it’s around the crossover frequency your problem is the phase matching is not right.

If your room is rectangular, try the Room EQ Wizard’s room simulator, see if you have similar results, and then you can tell how much a second sub would help.

My mains are Kef reference 3’s with a 2c center, Lyngdorf-50 AVR with a McIntosh MC207 powering everything.

I’ve tried moving my listening position forward and back, but I am then either too far forward or too far back.

When I used a 50 hz test tone, the center of the room had a 3 +/- db dip.  All sub frequencies were similar.  It really didn’t matter where I placed the sub, the void was still present in the center of the room. Some areas were worse but nowhere was great.

When I set up the sub I first ran DARO on the sub and then ran Room Perfect with the Lyngdorf. While all of this made things better, things are still not as good as they could be.