Please correct me if I am wrong but:
Unless the file has a mixed-down stereo version, then there WOULD be sonic information missing (bass drum?) if 5.1 is listened to in stereo only. No? Most SACD/Hybrids contain the "regular" version, the SACD version (that's playable on machines that can read it), and a 5.1 version (could be Dolby Digital).
The BSO high-rez downloads appear only to be 5.1 versions. From the site..."you must have a device capable of playing back surround sound audio. If you do not, the audio will only play back the front-two left/right channels." No DSD stereo mix-downs seem to be available.
It would be wise for companies, orchestras, to forget the manufacture of hard media like CDs, SACDs and others, downloads (but at a fair prices and non DRM) will be the future. If one wants a hard copy, burn it. Although I am not totally comfortable with this futuristic outlook—I still enjoy cleaning and playing vinyl, reading program notes in LARGE type, and yes, also feasting on all the new music available on CDs and SACDs—the enormous quality (if not quantity yet) of high-rez downloads available when you want them, appears to be the next big wave. If more companies like HDTracks and Linn Recordings appear and prosper, the prices could also become more competitive. The latter 2 companies' downloads are outstanding although there isn't much yet that catches my fancy. By the way, you can now buy some of the CSO Resound downloads at HDTracks.