If you allow for a small ad for our own products here ... ;-)
Vienna MIR offers individual "RoomTones" for each of its Venues. They are original recordings from the natural noise-floor in each of the halls, for every singly microphone array. Their levels are adjustable, of course, and they will even decode properly to the chosen output format (stereo, surround ...). The upcoming MIR Pro will offer the same feature with enhanced options (dedicated inserts and outputs for the RoomTone).
Kind regards,
/Dietz - Vienna Symphonic Library