low bass response

I would measure the room. Can just be test tones streamed and a simple DB meter. 
 

The only other thing I would add is distances between the front and side walls. You really need to be below 39” or over 7’ not to cancel out the low bass. This is due to the wave length at 1/4 of its length. Anyway try moving the speakers closer to the back and or side wall (driver less than 39” and less is better to not cancel low bass) and see what happens. Avoid the side wall and back all distances being equal. 
 

The revels have plenty of bass (I am a fan) but it is a little flat in the bass for my tastes. I personally run 2 subs regardless of the chosen speaker. I have yet to hear a speaker where high-passing the first octave (63hz and under) does not improve the sound. 
 

I'd say it's probably the listening position and/ or the speaker positioning within the room that is causing a perceived bass loss at your listening position.  A quick way to find out if this is the case is to simply move your listening chair 1-2 feet forward and backwards from where it is now, in 1-foot increments to see if the bass improves at all.  If so, then you know you're onto something. 

If you can increase the bass from the amp or the source instead of a flat curve increase the low frequencies a little higher 

I would agree with Erik_squires 

the wooden cabinet in between the speakers looks like a problem. Toe in your speakers and treat first reflection points as well as back walls.

GIK Acoustics provides a lot of excellent advice on their website. You shouldn’t need a sub for good bass. 
your system should sound fantastic, happy listening.
 

13 feet is very narrow.  There isn't room for enough air around the speakers.  Consider turning listening area 90 degrees and put speakers along one end of the longer wall.

Not sure subs are the answer either in this very small compromised space.

I would take the electronics out of the cupboard and put on conventional racks.  Move the cupboard to the other end of the room away from the sound action.  Use the emptied shelves for more LPs!!