System upgrade recommendation

what is not satisfying in your current set up that is causing you to consider upgrading?

a couple of thoughts, your receiver being a few years old may not have the best currently available room correction package and there may be other features on newer avr's that might benefit you.....also 2 subs are almost always better then 1....and I prefer matching subs when running 2 or more.

Does that receiver have Audyssey MultEQ? Or MultEQ XT or MultEQ 32? If the first, you could get some SQ bump by going to a receiver with a higher level of Audyssey.

Dual subs do sound better. They help to better smooth the bass response throughout the room.

Some simple room treatments can add phenomenal performance improvements.

Keep in mind, the loudspeakers/subs, and the acoustic environment in which they're placed, are the two overwhelmingly important elements in any system.

^^^ Can you give us a few pics of your setup? If not explain your setup and your room layout and what the walls, ceilings, and floors have on them.

The drawing is very helpful... I will let some of the room acoustics experts comment on room treatment or just simple everyday things you can do to make your room sound better.

In the positioning of your sub how did you choose that spot? Did you do the sub crawl? Or did you just put it there to use as an end table?
If you want to buy a 2nd EX I would suggest either placing 1 sub in the front right corner of the room and the other sub in the back left corner OR IF you really like having the sub as an end table for the on couch I would try the other sub as an end table for the other couch but have it on the end of the couch near the rear of the room like under your left side surround speaker.

A new AVR would also do good as well. With options like the Onkyo 818 that has Mult EQ XT 32 and all the new sound formats and tons of HDMI connections things will sound better and hook up with less clutter.

Do you sit in the seat in the bottom center of the image? If so, how close is it to the rear wall behind the seat? ,...ie, ears to wall in distance. You may benefit greatly from thick absorption there, if this is the case.

All the typical caveats apply; whatever bass traps you can get away with, mind the early/first reflections with absorption if needed.

Here's some great vids and reading to help out, these are great;

How to set up a room here.

RealTraps acoustics basics found here, is a simple and solid place to start.

Real Traps founder/owner and fellow AVS'er, Ethan Winer, has a tutorial page it is comprehensive.

Ethan has placed some outstanding videos on his site, the second one is of particular interst in this case.

It's not easy to tell from your image, however you may benefit from bringing your speakers in somewhat from the corners, and off the front wall a bit. Don't worry about narrowing the spread, getting them away from boundaries often increases the smoothness and image quality. Experiment, check out the links I provided, they're very good information. Good sound is all about the room. Regardless how expensive the gear is, it'll sound bad in a bad sounding room. The room dominates the sound.

Best of luck