My new speakers (ESP Boshran SE) produce an energy excess of 11 db at 200 hz in my room. The only placement that lowers this figure is with the speaker 8 inches from the wall and then there is no sound stage
This is typical of quarter wave cancellation.
The fact that placing your speakers up against the wall fixed it tends to confirm this (according to the article).
Another alternative - if you don't like the soundstage from very close to the wall placement - is to go far away from the wall or at least 6 to 8 feet.
Broadband absorption from acoustic treatment will ameliorate these problems at specific frequencies but it won't fix them (a few db). If you have a small square room 8 foot by 8 foot by 8 foot then turning it into a storage space might be the only option....
What the salesman neglected to mention was that the broadband absorption fixes all kinds of nulls throughout the mid/bass and midrange...so it makes lots of small improvements that can add up to a significant audible improvement.
IMHO a PARC is best for fixing things 120 Hz and below...and thick broadband acoustic absorption is best for 100 Hz and up.
At 200 Hz the wavelength is around 5 feet - trough to peak - so basically you can forget about fixing room modes with a PARC at these frequencies as what works at one position may be ineffective or make things worse a mere 2 feet away...