The perceived spectral balance of the Bose 901 is dominated by the spectral balance of the reflected sound, and IF the acoustic treatments in the room absorb the short wavelengths (high frequencies) moreso than they absorb the longer wavelengths, THEN the perceived spectral balance will be degraded accordingly.
To put it another way, if you go with acoustic treatments, I would suggest using products which deliberately PRESERVE the spectral balance of the reflected energy.
Duke