How to make small room sound bigger

Helmholtz diffusers are also powerful not only the very well known resonators which also diffuse some frequencies anyway...

It is a tube with a filtering end mouth with no neck and an open end... I used tube of different diameter and length...

By the way dont be afraid by the mathematical DESCRIPTION with equations...

Your ears/brain COMPUTE everything without using any numbers...

Just EXPERIMENT.... And LISTEN.....Most people are afraid of mathematic...learn the physical meaning of these devices , throw the equations on the garbage bin and experiment and listen with your own ears... Simple... It is FUN.... and it work... Especially if you use many devices that compensate each other... I created my device and tuned them by ears not by resolving a set of equations.... 😁😊

Is it perfect?

No...

Is it astounding in S.Q. improvement?

Yes, i laugh at the idea of any costly gear upgrade...

I listen music not sound now...

@mahgister - I am aware of Helmholtz resonators, I have read of their effectiveness but to be honest, I’d have a very steep learning curve on those. They certainly seem to be a lot more work than what I’ve done so far. Congratulations on your patience, because learning how to do it, before even implementation - wow.

While I know it's highly unlikely to perfect, I am looking for a way to maximize this, if at all possible in 11x14 room. 

This may sound nuts (oh heck, I've just chambered a bullet, someone's going to flame me over this no doubt).

I listen to orchestral music quite a lot, and I really love movie score. Try using your long wall as the front wall, try diffusion entirely on the front (long wall) with deep use of absorption in the front corners for lower frequencies. Absorption if possible on the top and bottom edges of your now long front wall.

Absorption on the back wall, however, if you're going to play with diffusion do not use quadratic (it's not going to give you benefit at all, certainly not here, it takes distance to develop an effect, which you do not have). You can get binary amplitude diffusion to work at extreme short distances, with absorption. If you do trial BAD with the holes, use 1/2" holes it helps with lower frequencies.

Set your speakers off the front wall, through trial and error and keep them in from the left and right boundaries ( I would start with 7 foot apart), this is why I am suggesting using the room the wrong way round. You could also try using absorption on first reflection points on the sides as well to open up for the larger sound you're trying to portray. The sides of the room, those reflections are a dead give away sonically on room size.

Your seating position would be not far off the back wall, giving you an equilateral triangle with your speakers, if you do the ceiling possibly try mixed absorption & diffusion.
If you have someone who can help you, a handy mirror to visually see the first off axis reflection points of both speakers (not just the closest one) on the side walls should be diffused or absorption treated.

It's absolutely a compromise, it's one that's worked for me and others, trying to get what you asked for.
 

@rixthetrick 

The maths is noted, but unfortunately only an approximation of the true equation so ,fortunately for me, not much help.

The problem with using a small room in 'landscape' orientation is that on the short dimension there isn't space behind the speakers and behind the listening position.  So if the speakers are moved forward to give them air from the front wall, as you say, the listener hears bad reflections from the rear wall just behind him.  It isn't easy to damp those.  It may well be better to accept some side wall reflections adjacent to the speakers.

Yes you have it - it's all a compromise.

@clearthinker - If he uses absorption, particularly deep or thick absorption behind and to the sides, it will help mask the back wall, or the lack of space from the back wall to some degree. In this or his case diffusion behind the listener is completely wasted I would imagine.

What I have suggested has worked in both a 26ft x 16ft room, and a 24ft x 15ft room with reasonable success. Both times this was used was because of entrance points and traffic into and out of the room, yes it’s a best fit compromise.
I wasn’t the clever person who came up with it, though after having actually heard it’s potential, implemented it for my own listening room. It has been working for me.

Now I am going to modify mine slightly as I do have an extra 4ft depth over the OP to try diffusion on my back wall, I’m going to need almost 6ft, the 4inches for the diffusers and 5.5ft for the effective working distance of the diffusers.

I listened again last night with a few albums, Daft Punk’s - Tron Legacy, The World of Hans Zimmer (live in concert) and the four disk set of Hans Zimmer movie scores (< that’s not the title sorry). It was not a room with the golden ratio and not perfect, but it was quite enjoyable with a decent sense of scale. I have been to venues with live orchestras and using that as a reference it truly wasn’t bad (I'm also not deluded, it will never be the same). I will say those big deep diffusers at the front work better than straight absorption ever could.

Being right up against the back wall, without diffusion there wasn’t much side reflections aimed at my ears, with speakers towed in a little, it wasn’t horrible.
The OP could move his seat all the way to the back wall and test this out for himself.

I have heard it, I do live with it, and it really can work with a bit of effort.
Failing having the room size, this has and can work the way I have suggested.

@ei001h - for goodness sakes get yourself a big thick and heavy (preferrably woolen) rug, maybe from an estate sale or used? The floor is really close to your speakers, that first reflection must be dealt with. Strictly absorption here, and not a little bit, a lot.

@rixthetrick, In this or his case diffusion behind the listener is completely wasted I would imagine. 

IMO absorption may be the preferred way to reduce the low range band levels and with direct radiated energy diffusion and absorption will be useful (needed). 

Low Freq will be the most difficult to treat and being the highest energy level the most important. As noted above placement is wide open for good results.

The Binary panels linked - look like a fine product and a quick glance at the Abfuser performance chart shows the effective range.

Congratulations on the QRD panels - well done!