Looking for an acoustic engineer or specialist

Search posts by Audiokinesis he's mentioned some good ones. Also recently posted a free for download book by Earl Geddes that is a total How To architectural design and construction tutorial on this very subject. This is all within the last few days so should be easy to find.

GIK acoustics

Art Noxon at Acoustic Sciences Corporation (ASC), the maker of the well known Tube Traps. Art does consultation work from his facility in Oregon, for both professional recording studios and home listening rooms. I've been in one of his rooms (that of Audiogon member folkfreak), and it is amazing.

Dennis Foley, Acoustic Fields, https://www.youtube.com/watch?v=uuSgzTk0lIE

First of all, kudos to you ebenlees, for setting out to really do it right!  The difference between what a professional acoustician can do and what an enthusiastic amateur acoustician can do is huge.  I fall into the latter category and after experiencing what a real professional can do, I have largely stopped giving out armchair-quarterback acoustic advice because mine falls so far short. 

I have worked with multi-award-winning acoustician Jeff Hedback on several projects, and his rooms are superb.  The first time I was in a room he had done, about ten years ago, when I closed my eyes it sounded like the room was twice its actual size.  I had never before experienced this disconnect between how big a room looked and how big it sounded.  Jeff is still affordable and he will work within your budget and aesthetic constraints as much as possible, and he is familiar with possibilities we haven't even thought  of.  Here is his website: 

http://www.hdacoustics.net/ 

Larry Borden of Dagogo interviewed Jeff a few years ago: 

https://www.dagogo.com/an-interview-with-jeff-hedback-of-hdacoustics/  

Duke