low bass response

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!!

You have the real-estate  so I would put the equipment  on the floor. On thick dense boards. On spikes. I would place the speakes on fits from the side wall and thirds from the end wall. Quarters  if you want mid bass. I would look at your receptacle s. I looked for bass in my new room I had nothing and I mean nothing changed out the industrial  20 a.p plugs for furutech  plugs started with the ones at the amps and I got some bass. Did the ones at the side where the rest of the equipment  is got a bunch more. I was shocked at the difference  in bass as well as transparency.  Guess your shelves us the receptacle s are causing the problem. Speaker placement  on thirds the speakers need to be almost 11 feet out into the room on foths you need to be six feet out into the room give them room to breathe.  A d yes treat first and second reflection  points I like mainly diffusion with just a small amount of suction I tent to like the sound of a live room and dislike  a dead over absorbed  room you will need a bit of absorption  though. Basically  in my opinion  you have to go find your bass you don't have to spend any huge amount  of money you need to find it. You have good gear and nothing needed there just have to find the bass. Powercords maybe the problem  as well. But I would look at that way  way down the list. 

Regards