This may give you some insight into Room Acoustics -
Primer: Acoustics - Absorption & Diffusion
https://www.avforums.com/threads/primer-acoustics-absorption-diffusion.1783772/
Though obviously all I've done is drawn on resources I tracked down on the Internet.
Is there any chance we can get photos of the space so we have a better idea of what we are dealing with? Also, we have the length of the room (9ft + 18ft = ~28ft), and we have a general sense of the high ceilings at 16ft. Also, flat or vaulted ceiling? But no sense of the width of the room.
Next we need a sense of the room acoustics. If this is very minimalist with bare wall, bare floors, and bare flat ceiling, leather furniture, etc.... or a more warm cluttered room with carpet, curtains, bookshelves, etc...?
The first consideration is the Point of First Reflection. These can typically be found with a mirror. Have someone slide a mirror along the side wall until you can see the speaker in the mirror. That is the point of first reflection.
As to the high ceiling, though in general that is to your advantage, and especially so if it is a vaulted or sloped ceiling. It depends on how the ceiling is finished. Flat Plaster, not so good, Acoustical tile or textured ceiling, better. You can hang what are referred to as
Sound Clouds from the ceiling. Really this is just an acoustical panel hanging horizontally from the ceiling. That would be the last thing I would consider, but it is something I would consider.
Depending on your budget, sound or acoustic panels need not look drab. Auralex and several other makers of acoustic panels have a long list of artists that have made their artwork available to be printed on the covering fabric of the panels. Or, many will print photos you've taken onto the panels.
http://sonic-print.com/
http://sonic-print.com/IndArtist.aspx
http://sonic-print.com/IA-AC/Ali_Cook.aspx
http://sonic-print.com/IA-GM/Greg_Menkedick.aspx
http://www.acoustimac.com/acousticart/
http://www.acoustimac.com/acousticart/art-categories
Acoustic panels definitely need not be drab.
While Absorption is important, too much can make a room dull. Equally important is Diffusion, or the breaking up and scattering of sound waves. More difficult to incorporate in a traditional room, but in a more modern room, they can come off as abstract art.
http://www.auralex.com/products/diffusion/
http://www.auralex.com/product/quadratec/
http://www.auralex.com/product/waveprism/
http://www.auralex.com/product/profusor/
You will also find many YouTube videos dealing with the subject of Room Acoustics. Both in the nature of room acoustics, the treatment of rooms, and making your own DIY Acoustic Panels or Diffusers.
In your case, the high ceilings and long room minimize the effect of room acoustics ... up to a point. So, you are in good shape to start with.
Steve/bluewizard