How to make small room sound bigger

I own a square room 13 feet by 13 and 8 feet ceiling...

Not an ideal one allegedly because a square one...

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My sound quality is so good that my 7 headphones are in a closet drawing for ever...

Passive material treatment is mandatory but it will not be enough to reach heaven...Only enough to improve the room ...To adapt the room response for the speakers it takes more than passive treatment...Especially in very small room...In fact i think now that the best room are small one, because we can use the time and timing aspect, and the reverberation factor to be positive from a psycho-acoustic standpoint...My room is like an intimate headphone sound, out of my head though, with sound filling the room... Speakers and walls disapear...

If you own a dedicated audio room not a living room, buy cheap discarded plumbers

tubes of various size and attach to them various straws of diffferent size diameter for adjustable neck in lenght and diameter and experiment with your EARS to tune the room....

If they are completely closed on one side they are "bottle" resonators.... Add to some tubes at one end some piece of fabric cloth which will act like some filter but keeping the mouth opened...Then you have diffusers....it is what i called mechanical control of the room...

All these tubes open on the two ends with one end filtered  or closed at one end with a neck are Helmholtz bottles resonators or diffusers in the firstcase .... NOTHING in passive treatment will replace this new distribution of pressure zones now synergetically tunable mechanically for your specific ears and specific speakers... Read the basic about Helmholtz resonators first...

Experiment and have fun...Dont be afraid, dont buy anything costly, EXPERIMENT....

Audiophile experience may cost peanuts, i know it by experience and experiments...Not by reading audio magazine... 😁😊

Mechanical control of the gear, electrical noise floor control of the house and especially acoustical control and not only walls passive treatment are the keys...

Price tag is meaningless in acoustic experience...Consumerism conditioning expanse is not musical experience, acoustic is...

The only trick to make room acoustically bigger is to open the door & the windows, and that allows longer wavelengths to form, and pushes the Schroeder frequency lower.

«Back in 1954, Schroeder referred to the frequency at which rooms go from being resonators to being reflectors/diffusors as the “crossover frequency.” We now call it the Schroeder frequency.»

It is the reason why using a grid of tuned Helmholtz resonators and diffusers, i smooth out any disruptive modes, i have around 100 of them from the diameter in millimeters to few inches with various lenght from 8 feet to 6 inches, i dont need a subwoofer with my 7 inches speakers box, and i enjoy all acoustical cues optimally for my room : dynamic, timbre,imaging, soundstage, listener envelopment, intimacy ETC...

Dont buy anything experiment and have fun...

Study basic acoustic WITHOUT any tool save your ears... Train them...

Acoustic phenomenon are not mainly linear mathematical phenomenon.... They are  mostly non linear ....You ears are the best tool IN YOUR ROOM....

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If you're a DIY kinda person there's a whole series of U tube videos from a German acoustic engineer by the name Jesco. They are excellent. The videos on GIK are also informative. If you're not interested in learning how to fix the acoustic short coming in your room and build treatments yourself, there's a whole world of companies that will be glad to help at a cost, but hey it's part of the hobby for me. Good luck, it can be a rabbit hole if you get into it.

sure modify your components  begin with power supply capacitors, install better resistors like Audio Note non-magnetic silver, add filter chokes to the AC and inputs, etc.  

Happy Listening.