room treatment gurus, please

Drywall with Cavities are the most difficult to work with. 

A Filled Cavity is the better of the Two Evils, the room coupling of Speakers is quite difficult in such an environment. 

It was described to me, by an individual who is quite skilled at producing Speakers, it is due to Drywall Systems with Cavities, that Typical Speaker Designs produced in the US are heavilyy augmented in the Bass Frequency.  The same individual stated that when such designs arrive to be a sale item in the UK, when used in a Typical UK Home the Bass is noticeably overbearing. 

A corner set up can be effective in a square room. You could also consult an acoustician. Jeff at hdacoustics is very good and I highly recommend him. Good luck!

You didn’t note the speaker and listening chair placement. I am stuck with a difficult room but have found inches of speaker movement can make a significant difference.  Same with moving the listening chair. It might be interesting to create a near-field space in the middle of the room temporarily (like at a mixing board) just to hear what’s possible. Also, the corner configuration mentioned above solves a lot of problems. 
 

Has your friend tried damping the rear wall to hold down standing waves?  Or any other experiments with the room?  Rugs on the floor?  A little bit of absorption can make big changes. Of course, if you can have an acoustician treat the room, all the better. I have found the experimentation interesting and kind of fun. It’s helpful to have friends with good ears who can provide feedback. 

You can still enjoy the system by using the right volume, not to overwhelm the room.Maybe you can use room divider that folds .

Having tried three types of room treatment in two awkward rooms and comparing these to a one room that is close to ideal. I tried Dirac (Datasat and others), Trinnov (both multi and 2 channel / 2.2 channel) and Lyngdorf "roomperfect".

Only Trinnov gave the level of performance and integration I was looking for (initially tried ST2 HiFi, then Amethyst). I have glass on two sides in awkward places in one of the rooms and on all for sides two with awkward places in another.  The difference between on correction on / off and completely bypassed (just to check against 'off) is huge.

But I acknowledge that some of the DSP benefit is that in both instances I am using speakers with smaller bass drivers than I would like and consequently I am integrating speaker arrays / sub integrations which would not be "an advantage" in a 2.0. 

However, where one of the sets of speakers I use are 2 way then I found the dsp crossovers were for both a single seat and especially for a multi-listening position more effective than either single or bi-amping without correction.   Comparing this to the set-up in the 'ideal' room gave a 'awkward room' performance that was quite close. Even the 'ideal', gained in timephase precision.

To be clear I am not in anyway suggesting that physical correction should not be undertaken or ignored.  But in cases where I simply could not remove windows, etc., Trinnov DSP did and excellent job.