Wilson Alexia 2 lacking some bass

I used to own Alexia 1's until moving up to Alexx V.  I tried adding subs to the Alexia's and it did not do much. Ultimately I think speaker position is key.

I also have the Alexia 2's.  I run it with Esoteric electronics.  The Alexia's bass is awesome is the word that comes to mind.  It's the sitting position that is the problem.  I found moving the sitting a couple of feet closer or further away from the speakers are areas of bass nulls.  Try it, move around your sitting position and see for yourself.

Suggest you try a good preamplifier in your system. My Bartok sounds great until I put the Conrad-Johnson ART88 pre back in the system. A much fuller more complete sound stage. Most, including now even Paul McGowan who fought the concept of a preamplifier, seem to find a more complete presentation with a pre in the system.

You may be sitting in a null. I had a room 15 x 20 x 8 and needed to move my seat back until the room response flattened out from my ear position. I did this 6 inches at a time with the laptop and mic until it was optimized. It was not at the 62% back rule of thumb. Roughly your 15 ft has a resonant freq of 37 hz and your 8 ft ceiling is 70. That is a problem right away. Your 20 feet is about 28 hz. That room had Sasha One's.

Today's room has Alexia One's. I had to bass trap the heck out of it to make it flatter. See my virtual room in Tennessee. The Sasha room is also in the system under Kentucky.

I think this is readily solvable. Your gear is stellar.

Set your speakers for best imaging get some Rel 812's