Bookshelf vs diffuser panel. Who wins?

@dekay  We have two libraries of books in my home (75% my collection/25% my late former wife's).   We took a 12' wall in a spare bedroom and installed book shelving 8' high to store about 1500 books (she was an historian so 30% of those books are of California/Los Angeles/local history).  There are books on comedians, seashells (she was a collector), architecture, art, U.S. history among the topics.  Then in our formal and large family room, we have beautiful built-in bookcases on either side of our fireplace (room has a 19' peak) with another 1000 art books, Judaica, entertainment (TV/Movie) books and leather bound fiction.   Another closet is filled with at least 500+ paperback fiction (I read my fill in my youth-prefer opera now).  I know, this doesn't have much to do with audio.  Read my next post. 

@ditusa However, take everything Acoustic Fields says as an attempt to separate you from your money.  I spent $150,000+ on constructing a listening room with the contractor who worked with him on other projects.  He never supplied the plans, they were in his head.  I had to pay by wire.  He put me off several months, then shipped incorrect sized carbon filters, too many acoustic filters boxes, ordered too much acoustiblok rolls and incorrect sized ceiling acoustic filter boxes.  After alerting him that the front and back wall box filters were killing the sound (overdamped), he said send me another $30K for front and rear quadradic diffusers.  Not they aren't worth it but they were going to be about 15" deep.   My room was only 19'6" deep.  I would have lost another 2'6" with a room depth of only 17'.  Yuck!   The exterior of the room was 22' , with 16" thick walls all around.  He cost me several months delay in time and materials.   

I use two pairs of Shakti Hallographs (2 in the front wall corners are certainly out of the way and two against the mid-walls).  I also use 34 Synergistic Research HFTs... The front wall HFTs... serve the function of 180 degree diffusers to a great extent.   I would like to add that the front 5'+ high HFT-X diffuser is the most critical to high-mid range balance and the four HFT 2.0s on the side of the speakers are the most critical for the bass vs. mid-treble balance.   The other 29 HFTs, not so much but they work together.  Hardly anyone mentioned their presence.  

I recommend quadradic diffusion if your room permits them.  Mine didn't.  

@jumia 

where is the book case located?  

side wall issues can be greatly minimized with speaker positioning.  

@fleschler 

Sorry to hear your story. Love to see your systems and venue.

You may have heard me talk about my room. I just really lucked out with incredible acoustics. Large highly asymmetric room. 

Guys: as with everything in life, there is no free lunch. Bookshelves, rags, towels, blankets… you name it. Been there done that. There is no substitute to real room treatments. Let’s not fool ourselves. Yes, we all want to save money AND effort/ work.