Speakers on the long wall

Op,

ABfuser and TAD are hybrid absorber/diffuser panels available on the market, but they tend to be expensive. You might want to consider a DIY approach using bookshelves. Consider multiple tall (84-inch), 5-shelf bookshelves along the back wall. Use the lower two shelves and stuff them with R23 Rockwool (6″ thick, ~$2.50/sq. ft.; I think it works better than fiberglass). You can cover the Rockwool with fabric or place decorative items in front so it doesn’t look unattractive—decorations can themselves act as partial reflectors. Use your imagination.

Use the upper three shelves for books, placing them in varied orientations. Again, be creative. To recap: the bookshelf serves as a free and effective diffuser. When the lower compartments are stuffed with thick R23 Rockwool, it can help absorb low frequencies while simultaneously deflecting mid-to-high frequencies at and above listening level. 

In a constrained space, the long wall setup is tougher, at least for me.  I feel for you.  Good luck and let us know what you end up achieved.

OP: I encountered a similar situation this year when I moved and underwent the process of setting up a new sound room.  The dimensions were almost identical to yours, mine being 15’ X 22’, with the ceiling being 7’8” high.

My speakers (AG Trios) were placed along the long wall.  The floor is heavy carpeting over concrete, the walls and ceiling are standard sheetrock.  There is an array of windows along the front wall.  The sound in this new setting was almost exactly as you described, disappointing!

This is how I fixed it.  First I put some gray triangular pillows from my old sound room in the corners.  I believe these were from Michael Green’s RoomTunes. Then I ordered diffusers for the back wall (the chair-side wall) from a company called Woodupp.  The specific panel I used is called acoustic wall slat panels.  They are inexpensive and look good cosmetically.  See photo.

This combination had brought my sound room back to life.  Good luck to you.

Looks like the photo from my post did not transfer.  Sorry.  I don't know how to post a photo on this forum.  I understand they cannot be uploaded directly from my computer and require a third party URL.  

By chance, are there any plans on how to design or make diffusers or absorbers?  Although when reality sinks in the other half will want something "not ugly".

I thought these diffusers were attractive and from a reputable company and not hideously expensive…

https://www.theabsolutesound.com/articles/2022-golden-ear-gik-acoustics-gotham-n23-5-quadratic-diffusers/

https://www.gikacoustics.com/products/gotham-primitive-root-diffuser

My room is 21x12 with a similar height ceiling with the same sort of listening distance from the long wall.The listening seat is a large leather sofa. Only one door to the left of the listening position. The ceiling is coved which improves the room accoustics

Speakers 12" from the wall and about 6.5' apart and TV in the middle.

The speakers are YG Hailey 2.2 and Vitus SIA 030.

It all sounds great and my suppliers have commented very favourably on the sound quality. One who is not a YG supplier tells me mine is the only system he has heard with YGs that impress him. He couldn't put his finger on it precisely but suspected it was the combination of the room and the Vitus amp

Hope that helps